Uv-vis radiation curable security inks for producing dichroic security features

ABSTRACT

The present invention provides a UV-Vis radiation curable security ink for producing a security feature for securing value documents, wherein said security feature exhibits a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light. The UV-Vis radiation curable security ink comprises a cationically curable or a hybrid curable ink vehicle, and silver nanoplatelets bearing a surface stabilizing agent of general formula (I)whereinthe residue RA is a C2-C4alkyl group substituted with a hydroxy group;the residue RB is selected from a C1-C4alkyl group, and a C2-C4alkyl group substituted with a hydroxy group; and Cat+ is an ammonium cation of general formula +NH2RCRD, wherein the residue RC is a C2-C4alkyl group substituted with a hydroxy group; and the residue RD is selected from a C1-C4alkyl group, and a C2-C4alkyl group substituted with a hydroxy group.

FIELD OF THE INVENTION

The present invention relates to the technical field of security inksfor producing dichroic security features for securing value documents,wherein said dichroic security features exhibit a first color uponviewing in transmitted light and a second color different from the firstcolor upon viewing in incident light.

BACKGROUND OF THE INVENTION

With the constantly improving quality of color photocopies and printingsand in an attempt to protect security documents such as banknotes, valuedocuments or cards, transportation tickets or cards, tax banderols, andproduct labels that have no reproducible effects against counterfeiting,falsifying or illegal reproduction, it has been the conventionalpractice to incorporate various security means features in thesedocuments.

Security features, e.g. for security documents, can generally beclassified into “covert” security features and “overt” securityfeatures. The protection provided by covert security features relies onthe concept that such features are difficult to detect, typicallyrequiring specialized equipment and knowledge for detection, whereas“overt” security features rely on the concept of being easily detectablewith the unaided human senses, e.g. such features may be visible and/ordetectable via the tactile senses while still being difficult to produceand/or to copy. However, the effectiveness of overt security featuresdepends to a great extent on their easy recognition as a securityfeature, because most users, and particularly those having no priorknowledge of the security features of a document or item securedtherewith, will only then actually perform a security check based onsaid security feature if they have actual knowledge of their existenceand nature.

A special role in securing value documents is played by dichroicsecurity features exhibiting a first color upon viewing in transmittedlight and a second color different from the first color upon viewing inincident light. To provide a striking effect and draw the layperson'sattention, the first color and the second color must have an attractivevisual appearance, such as blue, metallic yellow, magenta, and green,and a significant color contrast (for e.g.: blue/metallic yellow,green/metallic yellow, violet/metallic yellow).

Dichroic security features showing a blue color upon viewing intransmitted light and a metallic yellow color upon viewing in incidentlight may be obtained from silver platelets containing inks.

International patent application publication number WO2011064162A2describes solvent-based inks and UV radically curable inks comprisingsilver platelets for producing dichroic security, or decorative elementsshowing a gold/copper color in reflection and a blue color intransmission. Said inks contain high concentrations of silver plateletsbeing characterized by a weight ratio between the silver platelets andthe binder of 3:1.1. The high concentration of silver platelets in theinks used for obtaining the security, or decorative element described byWO2011064162A2 is detrimental to the mechanical resistance of theproduced security, or decorative element, and additionally, renders theproduction process of said element expensive. Further, the mechanicalresistance of the security, or decorative element, described byWO2011064162A2 is impaired by the use of UV radically curable inks orsolvent-based inks, which as well known to the skilled person, providecured coatings with limited mechanical resistance. As the mechanicalresistance is an essential property for security elements and themanufacturing process described by WO2011064162A2 is lengthy and ratherexpensive, the inks and the manufacturing process described therein arenot suitable for the industrial production on value documents ofdichroic security features with acceptable mechanical resistance.

International patent application publication number WO2013186167A2describes the use of a UV curable ink containing silver platelets, aradically curable binder and an important amount of organic solvent forcoating a surface of a holographic structure. The coated holographicstructure shows on the embossed surface a blue color with strong chromain transmission and a yellow color with a low chroma value inreflection. Although the UV curable ink described by WO2013186167A2contains a lower concentration of silver platelets when compared to theUV radiation radically curable ink described by WO2011064162A2, said inkis still not suitable for the industrial production of dichroic featureson value documents because on one side the increased amount of organicsolvent is not environmentally friendly and requires an additional airdrying step prior to the UV-curing step, and on the other side thecoatings obtained with said ink have limited mechanical resistance, aswell as low chroma in reflection.

Typically, industrial printing of value documents requires high printingspeeds of about 8,000 sheets/hour, wherein from each sheet an importantnumber of value documents is produced. For illustrative purpose, in thefield of banknotes printing, up to 55 value documents, each containingone or more security features, may be produced from one sheet. To besuitable for implementation on a production line, it is essential thatthe production process of each printable security feature present on avalue document complies with the high-speed requirements of industrialprinting of value documents.

Therefore, a need remains for stable security inks for producing onvalue documents at high speed (i.e. industrial speed) dichroic securityfeatures having improved mechanical resistance and exhibiting a bluecolor upon viewing in transmitted light and a metallic yellow color uponviewing in incident light.

SUMMARY OF THE INVENTION

Accordingly, it is the object of the present invention to provide UV-Visradiation cationically curable security inks and UV-Vis radiation hybridcurable security inks for producing on value documents at high speed(i.e. industrial speed) dichroic security features having improvedmechanical resistance and exhibiting a blue color upon viewing intransmitted light and a metallic yellow color upon viewing in incidentlight. This is achieved by the UV-VIS radiation curable security inkclaimed herein, wherein said ink comprises:

-   -   a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets        having a mean diameter in the range of 50 to 150 nm with a        standard deviation of less than 60%, a mean thickness in the        range of 5 to 30 nm with a standard deviation of less than 50%,        and a mean aspect ratio higher than 2.0, wherein the mean        diameter is determined by transmission electron microscopy and        the mean thickness is determined by transmission electron        microscopy, and        wherein the silver nanoplatelets bear a surface stabilizing        agent of general formula (I)

whereinthe residue R^(A) is a C₂-C₄alkyl group substituted with a hydroxygroup;the residue R^(B) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group; andCat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D), whereinthe residue R^(C) is a C₂-C₄alkyl group substituted with a hydroxygroup; and the residue R^(D) is selected from a C₁-C₄alkyl group, and aC₂-C₄alkyl group substituted with a hydroxy group;

-   -   b) from about 45 wt-% to about 80 wt-% of either a        cycloaliphatic epoxide, or a mixture of a cycloaliphatic epoxide        and one or more UV-Vis radiation curable compounds;    -   c) one or more cationic photoinitiators;    -   d) a perfluoropolyether surfactant functionalized with one or        more functional groups selected from the group consisting of:        hydroxyl, acrylate, methacrylate, and trialkoxysilyl;    -   e) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride        copolymer containing at least 60 wt-% of vinyl chloride; and        optionally    -   f) up to about 25 wt-% of an organic solvent;        the weight percents being based on the total weight of the        UV-Vis radiation curable security ink.

A further aspect according to the present invention is directed to aprocess for producing a security feature for securing a value document,wherein said security feature exhibits a blue color upon viewing intransmitted light and a metallic yellow color upon viewing in incidentlight, said process comprising the following steps:

-   -   A) printing, preferably by screen printing, rotogravure, or        flexography, the UV-Vis radiation curable security ink claimed        and described herein on a transparent or partially transparent        region of a substrate of a value document to provide an ink        layer; and    -   B) UV-Vis curing the ink layer obtained at step A) to form the        security feature.

DETAILED DESCRIPTION Definitions

The following definitions are to be used to interpret the meaning of theterms discussed in the description and recited in the claims.

As used herein, the article “a/an” indicates one as well as more thanone and does not necessarily limit its referent noun to the singular.

As used herein, the term “about” means that the amount or value inquestion may be the specific value designated or some other value in itsneighborhood. Generally, the term “about” denoting a certain value isintended to denote a range within ±5% of the value. As one example, thephrase “about 100” denotes a range of 100±5, i.e. the range from 95 to105. Preferably, the range denoted by the term “about” denotes a rangewithin ±3% of the value, more preferably ±1%. Generally, when the term“about” is used, it can be expected that similar results or effectsaccording to the invention can be obtained within a range of ±5% of theindicated value.

As used herein, the term “and/or” means that either all or only one ofthe elements of said group may be present. For example, “A and/or B”means “only A, or only B, or both A and B”. In the case of “only A”, theterm also covers the possibility that B is absent, i.e. “only A, but notB”.

The term “comprising” as used herein is intended to be non-exclusive andopen-ended. Thus, for instance a solution comprising a compound A mayinclude other compounds besides A. However, the term “comprising” alsocovers, as a particular embodiment thereof, the more restrictivemeanings of “consisting essentially of” and “consisting of”, so that forinstance “a solution comprising A, B, and optionally C” may also(essentially) consist of A, and B, or (essentially) consist of A, B, andC.

Where the present description refers to “preferred”embodiments/features, combinations of these “preferred”embodiments/features are also deemed to be disclosed as long as thespecific combination of “preferred” embodiments/features is technicallymeaningful.

As used herein, the term “one or more” means one, two, three, four, etc.

The term “UV-Vis curable” and “UV-Vis curing” refers to radiation-curingby photo-polymerization, under the influence of an irradiation havingwavelength components in the UV or in the UV and visible part of theelectromagnetic spectrum (typically 100 nm to 800 nm, preferably between150 and 600 nm and more preferably between 200 and 400 nm).

Surprisingly, it has been found that a UV-Vis radiation curable securityink comprising:

-   -   a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets        having a mean diameter in the range of 50 to 150 nm with a        standard deviation of less than 60%, a mean thickness in the        range of 5 to 30 nm with a standard deviation of less than 50%,        and a mean aspect ratio higher than 2.0, wherein the mean        diameter is determined by transmission electron microscopy and        the mean thickness is determined by transmission electron        microscopy, and        wherein the silver nanoplatelets bear a surface stabilizing        agent of general formula (I)

whereinthe residue R^(A) is a C₂-C₄alkyl group substituted with a hydroxygroup;the residue R^(B) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group; andCat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D),wherein the residue R^(C) is a C₂-C₄alkyl group substituted with ahydroxy group; and the residue R^(D) is selected from a C₁-C₄alkylgroup, and a C₂-C₄alkyl group substituted with a hydroxy group;

-   -   b) from about 45 wt-% to about 80 wt-% of either a        cycloaliphatic epoxide, or a mixture of a cycloaliphatic epoxide        and one or more UV-Vis radiation curable compounds;    -   c) one or more cationic photoinitiators;    -   d) a perfluoropolyether surfactant functionalized with one or        more functional groups selected from the group consisting of:        hydroxyl, acrylate, methacrylate, and trialkoxysilyl;    -   e) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride        copolymer containing at least 60 wt-% of vinyl chloride; and        optionally    -   f) up to about 25 wt-% of an organic solvent; the weight        percents being based on the total weight of the UV-Vis radiation        curable security ink, enables the expedient and cost-efficient        manufacturing of security features having improved mechanical        resistance and exhibiting a blue color in transmitted light and        a metallic yellow color in incident light. The combination of        the specific silver nanoplatelets bearing a surface stabilizing        agent of general formula (I) described herein and the specific        ink vehicle described herein allows expedient migration of the        silver nanoplatelets contained in an ink layer obtained by        printing the security ink according to the present invention        from the mass of the ink layer at the interface between the ink        layer and air and at the interface between the ink layer and the        substrate and alignment at said interfaces to form thin        reflective layers, thereby producing independently of the        thickness of the printed ink layer the metallic yellow color in        reflection and the blue color in transmission. The expedient        development of the metallic yellow color in reflection and of        the blue color in transmission cannot be achieved with the inks        described in the prior art. The cationically curable binder or        hybrid curable binder contained by the UV-Vis radiation curable        security ink claimed herein provides the dichroic security        feature obtained from said ink with a high mechanical        resistance. The attractive visual appearance and the contrast        between the blue color exhibited in transmitted light and the        metallic yellow color exhibited in incident light renders the        security feature made with the ink according to the present        invention conspicuous, thereby drawing the layperson's attention        to the security feature and aiding in finding and recognizing        said security feature on the value document, and in        authenticating the value document containing the security        feature. The UV-Vis radiation curable ink according to the        present invention has outstanding shelf stability. Hence, the        UV-Vis radiation curable security ink according to the present        invention is stable, complies with the high-speed requirements        of industrial printing of value documents and provides dichroic        security features with attractive visual appearance, high value        recognition and good mechanical resistance.

The security feature made with the UV-Vis radiation curable security inkclaimed herein exhibits a blue color upon viewing in transmitted lighti.e. in transmission. For the purposes of the present invention, viewingin transmitted light means that the security feature is illuminated fromone side, for example by holding said security feature against thedaylight or in front of a light source, and viewed from the oppositeside. Independently of the side from which the security feature isviewed in transmitted light, a blue color is observed. For the purposesof this invention, a security feature exhibiting a blue color refers toa security feature exhibiting a blue color characterized by a chromavalue C* (corresponding to a measure of the color intensity or colorsaturation) higher than 20. An intense to very intense blue color ischaracterized by a chroma value C* higher than 30. The chroma value C*is calculated from a* and b* values according to the CIELAB (1976) colorspace, wherein

C*=√{square root over ((a*)²+(b*)²)}.

Said a* and b* values in transmitted light are measured using aDatacolor 650 spectrophotometer (parameters: integration sphere, diffuseillumination (pulse xenon D65) and 8° viewing, analyzer SP2000 with dual256 diode array for wavelength range of 360-700 nm, transmissionsampling aperture size of 22 mm).

The security feature made with the UV-Vis radiation curable security inkclaimed herein exhibits a metallic yellow color or gold color uponviewing in incident light i.e. in reflection. In the present patentapplication, the terms “metallic yellow color” and “gold color” are usedinterchangeably. For the purpose of the present invention, “viewing inincident light” means that the security feature is illuminated from theside printed with the security ink claimed herein and viewed from thesame side. For the purpose of the present invention, a security featureexhibiting a metallic yellow color or gold color refers to a securityfeature exhibiting a yellow color characterized by a chroma value C*(corresponding to a measure of the color intensity or color saturation)higher than 20 as calculated from a* and b* values according to theCIELAB (1976) color space, wherein

C*=√{square root over ((a*)²+(b*)²)}

and wherein said a* and b* values of the security feature were measuredat 0° to the normal with an illumination angle of 22.5° using agoniometer (Goniospektrometer Codec WI-10 5&5 by Phyma GmbH Austria).

The UV-Vis radiation curable security ink claimed and described hereinis preferably selected from a screen-printing security ink, arotogravure security ink, and a flexography security ink. Preferably,the UV-Vis radiation curable security ink claimed herein ischaracterized by a viscosity of between about 50 mPas and about 2000mPas at 25° C. measured using a Brookfield viscometer (model “DV-IPrime) equipped with a spindle S27 at 100 rpm, or with a spindle S21 at50 rpm for measuring viscosities between 500 and 2000 mPas, and aspindle S21 at 100 rpm for measuring viscosities equal to or lower than500 mPas. The UV-Vis radiation curable screen-printing security inkclaimed herein is characterized by a viscosity of between about 50 mPasand about 1000 mPas at 25° C., preferably of between about 100 mPas andabout 1000 mPas at 25° C.

As known by those skilled in the art, the term rotogravure refers to aprinting process which is described for example in Handbook of PrintMedia, Helmut Kipphan, Springer Edition, page 48. Rotogravure is aprinting process wherein image elements are engraved into the surface ofthe cylinder. The non-image areas are at a constant original level.Prior to printing, the entire printing plate (non-printing and printingelements) is inked and flooded with ink. Ink is removed from thenon-image by a wiper or a blade before printing, so that ink remainsonly in the cells. The image is transferred from the cells to thesubstrate by a pressure typically in the range of 2 to 4 bars and by theadhesive forces between the substrate and the ink. The term rotogravuredoes not encompass intaglio printing processes (also referred in the artas engraved steel die or copper plate printing processes) which rely forexample on a different type of ink.

Flexography printing processes preferably use a unit with a chambereddoctor blade, an anilox roller and plate cylinder. The anilox rolleradvantageously has small cells whose volume and/or density determinesthe ink or varnish application rate. The chambered doctor blade liesagainst the anilox roller, filling the cells and scraping off surplusink or varnish at the same time. The anilox roller transfers the ink tothe plate cylinder which finally transfers the ink to the substrate.Plate cylinders can be made from polymeric or elastomeric materials.Polymers are mainly used as photopolymer in plates and sometimes as aseamless coating on a sleeve. Photopolymer plates are made fromlight-sensitive polymers that are hardened by ultraviolet (UV) light.Photopolymer plates are cut to the required size and placed in an UVlight exposure unit. One side of the plate is completely exposed to UVlight to harden or cure the base of the plate. The plate is then turnedover, a negative of the job is mounted over the uncured side and theplate is further exposed to UV light. This hardens the plate in theimage areas. The plate is then processed to remove the unhardenedphotopolymer from the non-image areas, which lowers the plate surface inthese non-image areas. After processing, the plate is dried and given apost-exposure dose of UV light to cure the whole plate. Preparation ofplate cylinders for flexography is described in Printing Technology, J.M. Adams and P. A. Dolin, Delmar Thomson Learning, 5^(th) Edition, pages359-360.

As well known to those skilled in the art, screen printing (alsoreferred in the art as silkscreen printing) is a printing technique thattypically uses a screen made of woven mesh to support an ink-blockingstencil. The attached stencil forms open areas of mesh that transfer inkas a sharp-edged image onto a substrate. A squeegee is moved across thescreen with ink-blocking stencil, forcing ink past the threads of thewoven mesh in the open areas. A significant characteristic of screenprinting is that a greater thickness of the ink can be applied to thesubstrate than with other printing techniques. Screen-printing istherefore also preferred when ink deposits with the thickness having avalue between about 10 to 50 μm or greater are required which cannot(easily) be achieved with other printing techniques. Generally, a screenis made of a piece of porous, finely woven fabric called mesh stretchedover a frame of e.g. aluminum or wood. Currently most meshes are made ofman-made materials such as synthetic or steel threads. Preferredsynthetic materials are nylon or polyester threads.

In addition to screens made on the basis of a woven mesh based onsynthetic or metal threads, screens have been developed out of a solidmetal sheet with a grid of holes. Such screens are prepared by a processcomprising of electrolytically forming a metal screen by forming in afirst electrolytic bath a screen skeleton upon a matrix provided with aseparating agent, stripping the formed screen skeleton from the matrixand subjecting the screen skeleton to an electrolysis in a secondelectrolytic bath in order to deposit metal onto said skeleton.

There are three types of screen-printing presses, namely flat-bed,cylinder and rotary screen-printing presses. Flat-bed and cylinderscreen printing presses are similar in that both use a flat screen and athree-step reciprocating process to perform the printing operation. Thescreen is first moved into position over the substrate, the squeegee isthen pressed against the mesh and drawn over the image area, and thenthe screen is lifted away from the substrate to complete the process.With a flat-bed press the substrate to be printed is typicallypositioned on a horizontal print bed that is parallel to the screen.With a cylinder press the substrate is mounted on a cylinder. Flat-bedand cylinder screen printing processes are discontinuous processes, andconsequently limited in speed which is generally at maximum 45 m/min inweb or 3,000 sheets/hour in a sheet-fed process.

Conversely, rotary screen presses are designed for continuous, highspeed printing. The screens used on rotary screen presses are forinstance thin metal cylinders that are usually obtained using theelectroforming method described hereabove or made of woven steelthreads. The open-ended cylinders are capped at both ends and fittedinto blocks at the side of the press. During printing, ink is pumpedinto one end of the cylinder so that a fresh supply is constantlymaintained. The squeegee is fixed inside the rotating screen andsqueegee pressure is maintained and adjusted to allow a good andconstant print quality. The advantage of rotary screen presses is thespeed which can reach easily 150 m/min in web or 10,000 sheets/hour in asheet-fed process.

Screen printing is further described for example in The Printing InkManual, R. H. Leach and R. J. Pierce, Springer Edition, 5^(th) Edition,pages 58-62, in Printing Technology, J. M. Adams and P. A. Dolin, DelmarThomson Learning, 5^(th) Edition, pages 293-328 and in Handbook of PrintMedia, H. Kipphan, Springer, pages 409-422 and pages 498-499.

More preferably the UV-VIS radiation curable security ink claimed anddescribed herein is a screen-printing security ink. Such UV-Visradiation curable screen-printing security ink is particularly usefulfor the industrial manufacturing of dichroic security features on valuedocuments because it enables printing at very high-speed of dichroicsecurity features having large thicknesses of at least about 4 μm.

The UV-VIS radiation curable ink claimed and described herein containsa) from about 7.5 wt-% to about 20 wt-%, preferably from about 7.5 wt-%to about 15 wt-%, more preferably from about 10 wt-% to about 13 wt-%,of silver nanoplatelets having a mean diameter in the range of 50 to 150nm with a standard deviation of less than 60%, a mean thickness in therange of 5 to 30 nm with a standard deviation of less than 50%, and amean aspect ratio higher than 2.0, wherein the mean diameter isdetermined by transmission electron microscopy and the mean thickness isdetermined by transmission electron microscopy, and wherein the silvernanoplatelets bear a surface stabilizing agent of general formula (I)

whereinthe residue R^(A) is a C₂-C₄alkyl group substituted with a hydroxygroup;the residue R^(B) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group; andCat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D),wherein the residue R^(C) is a C₂-C₄alkyl group substituted with ahydroxy group; andthe residue R^(D) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group.

The silver nanoplatelets described herein bearing the surfacestabilizing agent of general formula (I) are readily dispersible in thevehicle of the UV-Vis radiation curable security ink claimed herein.Upon printing, the silver nanoplatelets described herein migrate fromthe mass of the ink layer obtained with the UV-Vis radiation curablesecurity ink claimed herein at the interface between the ink layer andair and at the interface between the ink layer and the substrate andalign themselves to form a thin layer of silver nanoplatelets at saidinterfaces, thereby leading to the expedient development of the metallicyellow color observed in incident light. This property of the UV-Visradiation curable security ink claimed herein is particularlyadvantageous because on one side, the time required for the developmentof the metallic yellow color is compatible with the high-speedrequirements of industrial printing of value documents, and on the otherside, it enables production of dichroic security features with inkscontaining amounts of silver nanoplatelets as low as 7.5 wt-%, whichdrastically reduces the production costs, especially for dichroicsecurity features having a large thickness of at least about 4 μm.Depending on the thickness of the dichroic security feature to beproduced and the composition of the ink vehicle, the amount of thesilver nanoplatelets in the UV-Vis radiation curable security ink can beadjusted so that the metallic yellow color in reflected light is rapidlydeveloped without impacting the hue and chroma of the blue color intransmitted light.

The silver nanoplatelets contained by the UV-Vis may be in the form ofdisks, regular hexagons, triangles, especially equilateral triangles,and truncated triangles, especially truncated equilateral triangles, ormixtures thereof. They are preferably in the form of disks, truncatedtriangles, hexagons, or mixtures thereof.

The mean diameter of the silver nanoplatelets is in the range of 50 to150 nm, preferably 60 to 140 nm, more preferably 70 to 120 nm, with astandard deviation of less than 60%, preferably less than 50%. Thediameter of a silver nanoplatelet is the longest dimension of saidsilver nanoplatelet and corresponds to the maximum dimension of saidsilver nanoplatelet when oriented parallel to the plane of atransmission electron microscopy (TEM) image. As used herein, the term“mean diameter of the silver nanoplatelets” refers to the mean diameterdetermined by transmission electron microscopy (TEM) using Fiji imageanalysis software based on the measurement of at least 300 randomlyselected silver nanoplatelets oriented parallel to the plane of atransmission electron microscopy image (TEM), wherein the diameter of asilver nanoplatelet is the maximum dimension of said silver nanoplateletoriented parallel to the plane of a transmission electron microscopyimage (TEM). TEM analysis was conducted using an EM 910 instrument fromZEISS in bright field mode at an e-beam acceleration voltage of 100 kV.A dispersion of silver nanoplatelets in isopropanol at a suitableconcentration, preferably lower than 24.1 wt-%, was used for conductingthe TEM analysis.

The mean thickness of the silver nanoplatelets is in the range of 5 to30 nm, preferably 7 to 25 nm, more preferably 8 to 25 nm, with astandard deviation of less than 50%, preferably less than 30%. Thethickness of a silver nanoplatelet is the shortest dimension of saidnanoplatelet and corresponds to the maximum thickness of said silvernanoplatelet. As used herein, the term “mean thickness of the silvernanoplatelets” refers to the mean thickness determined by transmissionelectron microscopy (TEM) based on the manual measurement of at least 50randomly selected silver nanoplatelets oriented perpendicular to theplane of the TEM image, wherein the thickness of the silver nanoplateletis the maximum thickness of said silver nanoplatelet. TEM analysis wasconducted using an EM 910 instrument from ZEISS in bright field mode atan e-beam acceleration voltage of 100 kV. A dispersion of silvernanoplatelets in isopropanol at a suitable concentration, preferablylower than 24.1 wt-%, was used for conducting the TEM analysis.

The mean aspect ratio of the silver nanoplatelets (defined as the ratiobetween the mean diameter and the mean thickness) is larger than 2.0,preferably larger than 2.2 and more preferably larger than 2.5.

Preferably, the mean diameter of the silver nanoplatelets is in therange of 70 to 120 nm with the standard deviation being less than 50%,the mean thickness of said silver nanoplatelets is in the range of 8 to25 nm with the standard deviation being less than 30% and the meanaspect ratio of said silver nanoplatelets is higher than 2.5.

The silver nanoplatelets used in the UV-Vis radiation curable inkdescribed herein are characterized by a highest wavelength absorptionmaximum of between 560 and 800 nm, preferably 580 and 800 nm, mostpreferably 600 to 800 nm. The highest wavelength absorption maximum wasmeasured in water at ca. 5*10⁻⁵ M (mol/l) concentration of silver usinga Varian Cary 50 UV-Visible spectrophotometer. The absorption maximumhas a full width at half maximum (FWHM) value in the range of 50 to 500nm, preferably 70 to 450 nm, more preferably 80 to 450 nm. The molarextinction coefficient of the silver nanoplatelets, measured at thehighest wavelength absorption maximum, is higher than 4000L/(cm*mol_(Ag)), especially higher than 5000 L/(cm*mol_(Ag)), veryespecially higher than 6000 L/(cm*mol_(Ag)).

The silver nanoplatelets contained by the UV-Vis radiation curable inkclaimed herein bear a surface stabilizing agent of general formula (I)

wherein the residue R^(A) is a C₂-C₄alkyl group substituted with ahydroxy group; the residue R^(B) is selected from a C₁-C₄alkyl group,and a C₂-C₄alkyl group substituted with a hydroxy group; and Cat⁺ is anammonium cation of general formula ⁺NH₂R^(C)R^(D), wherein the residueR^(C) is a C₂-C₄alkyl group substituted with a hydroxy group; and theresidue R^(D) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group. Without being bound by thetheory, it is believed that the surface stabilizing agent of generalformula (I) besides preventing the agglomeration and sedimentation ofthe silver nanoplatelets in the security ink claimed herein, aids inpromoting migration of the silver nanoplatelets from the mass of the inklayer obtained with the security ink claimed herein at the interfacebetween the ink layer and air and at the interface between the ink layerand the substrate.

The surface stabilizing agent of general formula (I) may be present inan amount from about 0.5% to about 5%, preferably from about 0.5% toabout 4%, and more preferably in an amount of 3%, of the weight percent(wt-%) of the silver nanoplatelets.

The term “C₁-C₄alkyl group” as used herein refers to a saturated linearor branched-chain monovalent hydrocarbon radical of one to four carbonatoms (C₁-C₄). Examples of C₁-C₄alkyl groups include methyl (Me, —CH₃),ethyl (Et, —CH₂CH₃), 1-propyl (n-Pr, n-propyl, —CH₂CH₂CH₃), 2-propyl(i-Pr, iso-propyl, —CH(CH₃)₂), 1-butyl (n-Bu, n-butyl, —CH₂CH₂CH₂CH₃),2-methyl-1-propyl (i-Bu, i-butyl, —CH₂CH(CH₃)₂), 2-butyl (s-Bu, s-butyl,—CH(CH₃)CH₂CH₃) and 2-methyl-2-propyl (t-Bu, t-butyl, —C(CH₃)₃).

The term “C₂-C₄alkyl group substituted with a hydroxy group” refers to alinear or branched alkyl group having two to four carbon atoms, which issubstituted by a hydroxy group (—OH). The C₂-C₄alkyl group may besubstituted by one or two hydroxy groups.

In general formula (I), the residue R^(A) may be a C₂-C₄alkyl groupsubstituted with two hydroxy groups and the residue R^(B) may be a C₁-C₄alkyl group.

In a preferred embodiment according to the present invention, theresidues R^(A) and R^(B) are independently of each other a C₂-C₄alkylgroup substituted with a hydroxy group, preferably one hydroxy group.Thus, in an embodiment according to the present invention the residuesR^(A) and R^(B) are independently of each other selected from the groupconsisting of: —CH₂CH₂OH, —CH₂CH(OH)CH₃, —CH₂CH₂CH₂OH, —CH(CH₃)(CH₂OH),—CH₂CH(OH)CH₂CH₃, —CH₂CH₂CH(OH)CH₃, —CH₂CH₂CH₂CH₂OH, —CH(CH₃)CH(OH)CH₃,—CH(CH₂OH)CH₂CH₃, —CH(CH₃)CH₂CH₂OH, —CH₂CH(CH₂OH)CH₃, —CH₂C(CH₃)(OH)CH₃,—CH₂CH(CH₃)CH₂(OH), —CH₂C(OH)(CH₃)₂, —CH₂C(CH₃)(CH₂OH), more preferablyselected from the group consisting of: —CH₂CH₂OH, —CH₂CH(OH)CH₃, and—CH₂CH₂CH₂OH. The residues R^(A) and R^(B) may be the identical, or maybe different.

In general formula (I), the residue R^(C) may be a C₂-C₄alkyl groupsubstituted with two hydroxy groups and the residue R^(D) may be a C₁-C₄alkyl group.

In a preferred embodiment according to the present invention, theresidues R^(C) and R^(D) are independently of each other a C₂-C₄alkylgroup substituted with a hydroxy group, preferably one hydroxy group.Thus, in an embodiment according to the present invention the residuesR^(C) and R^(D) are independently of each other selected from the groupconsisting of: —CH₂CH₂OH, —CH₂CH(OH)CH₃, —CH₂CH₂CH₂OH, —CH(CH₃)(CH₂OH),—CH₂CH(OH)CH₂CH₃, —CH₂CH₂CH(OH)CH₃, —CH₂CH₂CH₂CH₂OH, —CH(CH₃)CH(OH)CH₃,—CH(CH₂OH)CH₂CH₃, —CH(CH₃)CH₂CH₂OH, —CH₂CH(CH₂OH)CH₃, —CH₂C(CH₃)(OH)CH₃,—CH₂CH(CH₃)CH₂(OH), —CH₂C(OH)(CH₃)₂, —CH₂C(CH₃)(CH₂OH), more preferablyselected from the group consisting of: —CH₂CH₂OH, —CH₂CH(OH)CH₃, and—CH₂CH₂CH₂OH. The residues R^(C) and R^(D) may be the identical, or maybe different.

Preferably, in general formula (I) the residues R^(A), R^(B), R^(C) andR^(D) are independently of each other a C₂-C₄alkyl group substitutedwith one hydroxy group. More preferably, in general formula (I) theresidues R^(A), R^(B), R^(C) and R^(D) are independently of each otherselected from the group consisting of: —CH₂CH₂OH, —CH₂CH(OH)CH₃, and—CH₂CH₂CH₂OH. Even more preferably, in general formula (I) the residuesR^(A), R^(B), R^(C) and R^(D) represent —CH₂CH₂OH.

To prevent agglomeration and sedimentation of the silver nanoplateletsupon storage, the silver nanoplatelets may bear on their surface furthersurface stabilizing agents.

In a preferred embodiment, the silver nanoplatelets bear on theirsurface a further surface stabilizing agent of general formula (II)

-   -   wherein    -   R¹ is H, C₁-C₁₈alkyl, phenyl, C₁-C₈alkylphenyl, or CH₂COOH;    -   R², R³, R⁴, R⁵, R⁶ and R⁷ are independently of each other H,        C₁-C₈alkyl, or phenyl;    -   Y is O, or NR⁸;    -   R⁸ is H, or C₁-C₈alkyl;    -   k1 is an integer in the range of from 1 to 500;    -   k2 and k3 are independently of each other 0, or integers in the        range of from 1 to 250;    -   k4 is 0, or 1; and    -   k5 is an integer in the range of from 1 to 5. Preferably, in        general formula (II) Y represents O. Also preferably, in general        formula (II) k4 is 0.

The surface stabilizing agent of general formula (II) has preferably anaverage molecular weight (Mn) of from 1000 to 20000 [g/mol], and morepreferably from 1000 to 10000 [g/mol], most preferably from 1000 to 6000[g/mol].

If the surface stabilizing agent of formula (I) comprises, for example,ethylene oxide units (EO) and propylene oxide units (PO), the order of(EO) and (PO) may be fixed (block copolymers), or may not be fixed(random copolymers).

Preferably, in general formula (II), R¹ is H, or C₁-C₁₈alkyl, R², R³,R⁴, R⁵, R⁶ and R⁷ are independently of each other H, CH₃, or C₂H₅, k1 isan integer in the range of from 22 to 450, k2 and k3 are independentlyof each other 0, or integers in the range of from 1 to 250, k4 is 0, or1, and k5 is an integer in the range of from 1 to 5. More preferably, ingeneral formula (II) R¹ is H, or C₁-C₄alkyl, R², R³, R⁴, R⁵, R⁶ and R⁷are independently of each other H, or CH₃, k1 is an integer in the rangeof from 22 to 450, k2 and k3 are independently of each other 0, orintegers in the range of from 1 to 100, k4 is 0, k5 is an integer in therange of from 1 to 4.

The most preferred surface stabilizing agent of general formula (II) hasthe general formula (II-a)

-   -   wherein    -   R¹ is H, or a C₁-C₈alkyl group, especially H, or CH₃, and    -   k1 is an integer in the range of from 22 to 450, especially 22        to 150.

The preferred surface stabilizing agents of general formula (II) arederived from MPEG thiols (poly(ethylene glycol) methyl ether thiols)having an average molecular weight (M_(n)) of 2000 to 6000, such as, forexample, MPEG 2000 thiol, MPEG 3000 thiol, MPEG 4000 thiol, MPEG 5000thiol, MPEG 6000 thiol, PEG thiols (O-(2-mercaptoethyl)-poly(ethyleneglycol)) having an average M_(n) of 2000 to 6000, such as, for example,PEG 2000 thiol, PEG 3000 thiol, PEG 4000 thiol, PEG 5000 thiol, PEG 6000thiol.

The silver nanoplatelets contained by the security ink may further beara surface stabilizing agent which is a polymer, or copolymer describedin WO200674969A1, which can be obtained by a process comprising thesteps:

-   -   i-1) polymerizing in a first step one or more ethylenically        unsaturated monomers in the presence of at least one        nitroxylether having the structural element

wherein X represents a group having at least one carbon atom and is suchthat the free radical X• derived from X is capable of initiatingpolymerization; or

-   -   i-2) polymerizing in a first step one or more ethylenically        unsaturated monomers in the presence of at least one stable free        nitroxyl radical

and a free radical initiator;

-   -   wherein at least one monomer used in the steps i-1) or i-2) is a        C₁-C₆alkyl or hydroxy C₁-C₆alkyl ester of acrylic or methacrylic        acid; and optionally    -   ii) a second step, comprising the modification of the polymer or        copolymer prepared under i-1) or i-2) by a transesterification        reaction, an amidation, hydrolysis or anhydride modification or        a combination thereof.

The monomer in step i-1) or i-2) is preferably selected from4-vinyl-pyridine or pyridinium-ion, 2-vinyl-pyridine or pyridinium-ion,1-vinyl-imidazole or imidazolinium-ion, or a compound of formulaCH₂═C(R_(a))—(C═Z)—R_(b), wherein

-   -   R_(a) is hydrogen or methyl;    -   R_(b) is NH₂, O⁻(Me⁺), unsubstituted C₁-C₁₈alkoxy, C₂-C₁₀₀alkoxy        interrupted by at least one N and/or O atom, or        hydroxy-substituted C₁-C₁₈alkoxy, unsubstituted        C₁-C₁₈alkylamino, unsubstituted di(C₁-C₁₈alkyl)amino,        hydroxy-substituted C₁-C₁₈alkylamino or hydroxy-substituted        di(C₁-C₁₈alkyl)amino, —O(CH₂)_(y)NR¹⁵R¹⁶ or        —O(CH₂)_(y)N⁺HR¹⁵R¹⁶An⁻, —N(CH₂)_(y)NR¹⁵R¹⁶, or        —N(CH₂)_(y)N⁺HR¹⁵R¹⁶An⁻,    -   wherein    -   An⁻ is an anion of a monovalent organic, or inorganic acid;    -   y is an integer from 2 to 10;    -   R¹⁵ is saturated or unsaturated, linear or branched chain alkyl        with 1-22 carbon atoms;    -   R¹⁶ is saturated or unsaturated, linear or branched chain alkyl        with 1-22 carbon atoms;    -   Me⁺ is a monovalent metal atom or the ammonium ion; and    -   Z is oxygen or sulfur.

The second step ii) is preferably a transesterification reaction. Instep ii) the alcohol is preferably an ethoxylate of formulaR_(c)—[O—CH₂—CH₂—]_(c)—OH, wherein R_(c) is saturated or unsaturated,linear or branched chain alkyl with 1-22 carbon atoms, or alkylaryl ordialkylaryl with up to 24 carbon atoms and c is 1 to 150.

Preferably, step i-1) or i-2) is carried out twice and a block copolymeris obtained wherein in the first or second radical polymerization stepthe monomer or monomer mixture contains 50 to 100% by weight, based ontotal monomers, of a C₁-C₆ alkyl ester of acrylic or methacrylic acidand in the second or first radical polymerization step respectively, theethylenically unsaturated monomer or monomer mixture contains at least amonomer without primary or secondary ester bond.

In the first polymerization step, the monomer or monomer mixturecontains from 50 to 100% by weight based on total monomers of aC₁-C₆alkyl ester of acrylic or methacrylic acid (first monomer) and inthe second polymerization step the ethylenically unsaturated monomer ormonomer mixture comprises 4-vinyl-pyridine or pyridinium-ion,2-vinyl-pyridine or pyridinium-ion, vinyl-imidazole orimidazolinium-ion, 3-dimethylaminoethylacrylamide,3-dimethylaminoethylmethacrylamide, or corresponding ammonium ion,3-dimethylaminopropylacrylamide, or corresponding ammonium ion, or3-dimethylaminopropylmethacrylamide, or corresponding ammonium ion(second monomer).

Preferably, the nitroxylether has the following structure

The surface stabilization agent is preferably a copolymer which can beobtained by a process comprising the steps:

-   -   i-2) polymerizing in a first step a first monomer, which is a        C₁-C₆alkyl or hydroxy C₁-C₆alkyl ester of acrylic or methacrylic        acid, and a second monomer which is selected from selected from        4-vinyl-pyridine or pyridinium-ion, 2-vinyl-pyridine or        pyridinium-ion, 1-vinyl-imidazole or imidazolinium-ion,        3-dimethylaminoethylacrylamide,        3-dimethylaminoethylmethacrylamide,        3-dimethylamino-propylacrylamide, and        3-dimethylaminopropylmethacrylamide; in the presence of at least        one nitroxylether having the structural element

-   -   and    -   ii) a second step, comprising the modification of the polymer or        copolymer prepared under i-1) by a transesterification reaction,        wherein the alcohol in step ii) is an ethoxylate of formula        R_(c)—[O—CH₂—CH₂—]_(c)—OH, wherein R_(c) is saturated or        unsaturated, linear or branched chain alkyl with 1-22 carbon        atoms, or alkylaryl or dialkylaryl with up to 24 carbon atoms        and c is 1 to 150.

Preferably the surface stabilizing agent obtained via the processdescribed herein is a copolymer of the following formula (III)

-   -   wherein    -   R^(17a), R^(17b) and R^(17c) are independently of each other H,        or methyl;    -   R^(18a) and R^(18b) are H, or methyl;    -   R^(19a) is saturated or unsaturated, linear or branched chain        alkyl with 1-22 carbon atoms;    -   R^(19b) is R_(c)—[O—CH₂—CH₂—]_(c)—O—;    -   R^(19c) is

—C(═O)—NH—(CH₂)_(y)N⁺HR¹⁵R¹⁶An⁻;

-   -   wherein    -   An⁻ is an anion of a monovalent organic, or inorganic acid;    -   y is an integer from 2 to 10;    -   R¹⁵ is saturated or unsaturated, linear or branched chain alkyl        with 1-22 carbon atoms,    -   R¹⁶ is saturated or unsaturated, linear or branched chain alkyl        with 1-22 carbon atoms,    -   R_(c) is saturated or unsaturated, linear or branched chain        alkyl with 1-22 carbon atoms, or alkylaryl or    -   dialkylaryl with up to 24 carbon atoms and c is 1 to 150, and    -   y1, y2 and y3 are independently of each other integers from 1        to 200. In general formula (III) the order of monomers with        indices y1 and y2 may be fixed (block copolymers) or not fixed        (random copolymers).

Surface stabilizing agents of general formula (III) have been describedin the international patent application publication numberWO200674969A1.

A preferred surface stabilizing agent of general formula (III) is acompound of general formula (III-a)

-   -   wherein    -   R^(18a) and R^(18b) are H, or methyl;    -   y1, y2 and y3 are independently of each other integers from 1 to        200; and    -   c is an integer from 1 to 150. The order of monomers with        indices y1 and y2 may be fixed (block copolymers) or not fixed        (random copolymers).

Examples of preferred copolymers to be used as surface stabilizingagents are the copolymers described in Example A3 and Example A6 ofWO200674969A1.

To improve the stability of optical properties of the silvernanoplatelets upon storage or heat exposure, said silver nanoplateletsmay bear a further surface stabilizing agent of general formula (IV)

-   -   wherein    -   R⁹ is a hydrogen atom, or a group of formula —CHR¹¹—N(R¹²)(R¹³);    -   R¹⁰ is a hydrogen atom, a halogen atom, a C₁-C₈alkoxy group, or        a C₁-C₈alkyl group;    -   R¹¹ is H, or C₁-C₈alkyl; and    -   R¹² and R¹³ are independently of each other a C₁-C₈alkyl, a        hydroxyC₁-C₈alkyl group, or a group of formula        —[(CH₂CH₂)—O]_(n1)—CH₂CH₂—OH, wherein n1 is 1 to 5.

Examples of compounds of formula (IV) include, but are not limited to:

A dispersion of silver nanoplatelets to be used for preparing the UV-Visradiation curable security ink claimed herein may be obtained by usingthe method comprising the following steps:

-   -   1) preparing a solution comprising a silver precursor, a        compound of formula (II)

-   -   wherein    -   R¹ is H, C₁-C₁₈alkyl, phenyl, C₁-C₈alkylphenyl, or CH₂COOH;    -   R², R³, R⁴, R⁵, R⁶ and R⁷ are independently of each other H,        C₁-C₈alkyl, or phenyl;    -   Y is O, or NR⁸;    -   R⁸ is H, or C₁-C₈alkyl;    -   k1 is an integer in the range of from 1 to 500;    -   k2 and k3 are independently of each other 0, or integers in the        range of from 1 to 250;    -   k4 is 0, or 1; and    -   k5 is an integer in the range of from 1 to 5;    -   a polymer, or copolymer, which can be obtained by a process        comprising the steps:    -   i-1) polymerizing in a first step one or more ethylenically        unsaturated monomers in the presence of at least one        nitroxylether having the structural element

wherein X represents a group having at least one carbon atom and is suchthat the free radical X• derived from X is capable of initiatingpolymerization; or

-   -   i-2) polymerizing in a first step one or more ethylenically        unsaturated monomers in the presence of at least one stable free        nitroxyl radical

and a free radical initiator; wherein at least one monomer used in thesteps i-1) or i-2) is a C₁-C₆ alkyl or hydroxy C₁-C₆ alkyl ester ofacrylic or methacrylic acid; and optionally

-   -   ii) a second step, comprising the modification of the polymer or        copolymer prepared under i-1) or i-2) by a transesterification        reaction, an amidation, hydrolysis or anhydride modification or        a combination thereof,        water, and optionally a defoamer;    -   2) preparing a solution, comprising a reducing agent, which        comprises at least one boron atom in the molecule, and water;    -   3) adding the solution obtained at step 1) to the solution        obtained at step 2), and adding one or more complexing agents;    -   4) adding a hydrogen peroxide solution in water; and    -   5) adding one or more surface stabilizing agents to the mixture        obtained at step 4).

The silver precursor is a silver(I) compound selected from the groupconsisting of: AgNO₃; AgClO₄; Ag₂SO₄; AgCl; AgF; AgOH; Ag₂O; AgBF₄;AglO₃; AgPF₆; R²⁰⁰CO₂Ag, R²⁰⁰SO₃Ag, wherein R²⁰⁰ is unsubstituted orsubstituted C₁-C₁₈alkyl, unsubstituted or substituted C₅-C₈cycloalkyl,unsubstituted or substituted C₇-C₁₈aralkyl, unsubstituted or substitutedC₆-C₁₈aryl or unsubstituted or substituted C₂-C₁₈heteroaryl; Ag salts ofdicarboxylic, tricarboxylic, polycarboxylic acids, polysulfonic acids,P-containing acids and mixtures thereof, preferably from the groupconsisting of: silver nitrate, silver acetate, silver perchlorate,silver methanesulfonate, silver benzenesulfonate, silvertoluenesulfonate silver trifluoromethanesulfonate, silver sulfate,silver fluoride and mixtures thereof, and more preferably is silvernitrate.

The reducing agent is selected from the group consisting of alkali, oralkaline earth metal borohydrides, such as sodium borohydride, alkali,or alkaline earth metal acyloxyborohydrides, such as sodiumtriacetoxyborohydride, alkali, or alkaline earth metal alkoxy- oraryloxyborohydrides, such as sodium trimethoxyborohydride,aryloxyboranes, such as catecholborane, and amine-borane complexes, suchas diethylaniline borane, tert-butylamine borane, morpholine borane,dimethylamine borane, triethylamine borane, pyridine borane, ammoniaborane and mixtures thereof. Sodium borohydride is most preferred.

The one or more complexing agents are selected from the group ofchlor-containing compounds, which are capable to liberate chloride ionsunder reaction conditions, such as metal chlorides, alkyl or arylammonium chlorides, phosphonium chlorides; primary or secondary aminesand corresponding ammonium salts, such as methyl amine or dimethylamine;ammonia and corresponding ammonium salts; and aminocarboxylic acids andtheir salts, such as ethylenediaminetetraacetic acid.

Non limiting examples of complexing agents include ammonia, methylamine,dimethylamine, ethylamine, ethylenediamine, diethylenetriamine,ethylene-diamine-tetraacetic acid (EDTA); ethylenediamineN,N′-disuccinic acid (EDDS); methyl glycine diacetic acid (MGDA);diethylene triamine pentaacetic acid (DTPA); propylene diaminetetracetic acid (PDTA); glutamic acid N,N-diacetic acid(N,N-dicarboxymethyl glutamic acid tetrasodium salt (GLDA);nitrilotriacetic acid (NTA), and any salts thereof;N-hydroxyethylethylenediaminetri-acetic acid (HEDTA),triethylenetetraaminehexaacetic acid (TTHA), N-hydroxyethyliminodiaceticacid (HEIDA), dihydroxyethylglycine (DHEG),ethylenediaminetetrapropionic acid (EDTP) and derivatives thereof, suchas, for example, trisodium salt of methylglycinediacetic acid (Na₃MGDA)and tetrasodium salt of EDTA.

The defoamer is a compound or composition, capable to suppress foamformation in the reaction mixture, such as, for example, commerciallyavailable TEGO® Foamex 1488, 1495, 3062, 7447, 800, 8030, 805, 8050,810, 815N, 822, 825, 830, 835, 840, 842, 843, 845, 855, 860, 883, K 3, K7, K 8, N, Antifoam SE-15 from Sigma, Struktol SB-2080 and the like. Theamount of the defoamer is in the range of from 0.00001% to 5% by weightbased on total weight of reaction mixture prior to hydrogen peroxideaddition, preferably from 0.0001% to 3% and more preferably from 0.001%to 2% by weight.

The defoamer can be added to the solution prepared at step 1) and/or tothe solution prepared at step 2).

The reaction of silver nanoplatelets formation is carried out bygradually adding the silver precursor solution to the reducing agentsolution, whereas the temperature of both solutions is in the range of−3° C. to 40° C. and the gradual addition is completed within 15 minutesto 24 h time.

The silver nanoplatelets obtained at step 4) and/or 5) can be submittedto further purification and/or isolation methods, such as decantation,(ultra)filtration, (ultra)centrifugation, reversible or irreversibleagglomeration, phase transfer with organic solvent, and combinationsthereof. The dispersion of silver nanoplatelets may contain up to about99 wt-% silver nanoplatelets, preferably from 5 wt-% to 99 wt-% silvernanoplatelets, more preferably from 5 wt-% to 90 wt-% silvernanoplatelets, the wt-% being based on the total weight of thedispersion.

Starting from the silver nanoplatelets obtained by purification and/orisolation, the silver nanoplatelets bearing the surface stabilizingagent of general formula (I) can be prepared by:

i) reacting CS₂ with an amine of formula R^(A)R^(B)NH in the presence ofthe silver nanoplatelets and subsequent treatment with R^(C)R^(D)NH.orii) by reacting CS₂ with an amine of formula R^(A)R^(B)NH and subsequenttreatment with R^(C)R^(D)NH to obtain the dithiocarbamate of generalformula (I), which is then reacted with the silver nanoplatelets.

Silver nanoplatelets bearing a dithiocarbamate of general formula (I),wherein R^(A) is identical with R^(C) and R^(B) is identical with R^(D)can be obtained starting from the silver nanoplatelets subjected topurification and/or isolation methods:

iii) by reacting CS₂ with an amine of formula R^(A)R^(B)NH in thepresence of the silver nanoplatelets; or iv) by reacting CS₂ with anamine of formula R^(A)R^(B)NH to obtain the dithiocarbamate of generalformula (I)

which is then reacted with the silver nanoplatelets.

The silver nanoplatelets described herein are disclosed by the Europeanpatent application number 20206698.1 entitled “Compositions, comprisingsilver nanoplatelets” filed by BASF SE on Nov. 10, 2020.

The UV-Vis radiation curable security ink claimed herein contains b)from about 45 wt-% to about 80 wt-% of either a cycloaliphatic epoxide,or a mixture of a cycloaliphatic epoxide and one or more UV-Visradiation curable compounds. The one or more UV-Vis radiation curablecompounds may comprise one or more cationically curable monomers, and/orone or more radically curable monomers and/or oligomers. If the one ormore UV-Vis radiation curable compounds comprise one or more radicallycurable monomers and/or oligomers, then the UV-Vis radiation curablesecurity ink claimed herein further comprises g) one or more freeradical photoinitiators. Thus, the present invention is directed to aUV-Vis radiation curable security ink for producing a security featureexhibiting a blue color upon viewing in transmitted light and a metallicyellow color upon viewing in incident light, wherein said ink comprises:

-   -   a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets        having a mean diameter in the range of 50 to 150 nm with a        standard deviation of less than 60%, a mean thickness in the        range of 5 to 30 nm with a standard deviation of less than 50%,        and a mean aspect ratio higher than 2.0, wherein the mean        diameter is determined by transmission electron microscopy and        the mean thickness is determined by transmission electron        microscopy, and        wherein the silver nanoplatelets bear a surface stabilizing        agent of general formula (I)

-   -   wherein    -   the residue R^(A) is a C₂-C₄alkyl group substituted with a        hydroxy group;    -   the residue R^(B) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group; and    -   Cat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D),    -   wherein    -   the residue R^(C) is a C₂-C₄alkyl group substituted with a        hydroxy group; and    -   the residue R^(D) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group;    -   b) b-1) from about 45 wt-% to about 80 wt-% of a cycloaliphatic        epoxide;        -   b-2) from about 45 wt-% to about 80 wt-% of a mixture of a            cycloaliphatic epoxide and one or more cationically curable            monomers;        -   b-3) from about 45 wt-% to about 80 wt-% of a mixture of a            cycloaliphatic epoxide and one or more radically curable            monomers and/or oligomers; or        -   b-4) from about 45 wt-% to about 80 wt-%, preferably from            about 45 wt-% to about 65 wt-%, of a mixture of a            cycloaliphatic epoxide, one or more cationically curable            monomers and one or more radically curable monomers and/or            oligomers;    -   c) one or more cationic photoinitiators;    -   d) a perfluoropolyether surfactant functionalized with one or        more functional groups selected from the group consisting of:        hydroxyl, acrylate, methacrylate, and trialkoxysilyl;    -   e) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride        copolymer containing at least 60 wt-% of vinyl chloride; and        optionally    -   f) up to about 25 wt-% of an organic solvent;        with the proviso that if the security ink comprises b-3) or        b-4), the security ink further comprises g) one or more free        radical photoinitiators; the weight percents being based on the        total weight of the UV-Vis radiation curable security ink.

A preferred embodiment according to the present invention is directed toa UV-Vis radiation cationically curable security ink (i.e. an inkcontaining exclusively cationically curable monomers and no radicallycurable monomers/oligomers) for producing a security feature exhibitinga blue color upon viewing in transmitted light and a metallic yellowcolor upon viewing in incident light, wherein said ink comprises:

-   -   a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets        having a mean diameter in the range of 50 to 150 nm with a        standard deviation of less than 60%, a mean thickness in the        range of 5 to 30 nm with a standard deviation of less than 50%,        and a mean aspect ratio higher than 2.0, wherein the mean        diameter is determined by transmission electron microscopy and        the mean thickness is determined by transmission electron        microscopy, and    -   wherein the silver nanoplatelets bear a surface stabilizing        agent of general formula (I)

-   -   wherein    -   the residue R^(A) is a C₂-C₄alkyl group substituted with a        hydroxy group;    -   the residue R^(B) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group; and    -   Cat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D)        wherein    -   the residue R^(C) is a C₂-C₄alkyl group substituted with a        hydroxy group; and    -   the residue R^(D) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group;    -   b) b-1) from about 45 wt-% to about 80 wt-% of a cycloaliphatic        epoxide; or        -   b-2) from about 45 wt-% to about 80 wt-% of a mixture of a            cycloaliphatic epoxide and one or more cationically curable            monomers;    -   c) one or more cationic photoinitiators;    -   d) a perfluoropolyether surfactant functionalized with one or        more functional groups selected from the group consisting of:        hydroxyl, acrylate, methacrylate, and trialkoxysilyl;    -   e) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride        copolymer containing at least 60 wt-% of vinyl chloride; and        optionally    -   f) up to about 25 wt-% of an organic solvent; the weight        percents being based on the total weight of the UV-Vis radiation        cationically curable security ink. If the UV-Vis radiation        cationically curable security ink contains b-2) from about 45        wt-% to about 80 wt-% of a mixture of a cycloaliphatic epoxide        and one or more cationically curable monomers, it is preferred        that the ratio between the total weight percent (wt-%) of the        one or more cationically curable monomers and the weight percent        (wt-%) of the cycloaliphatic epoxide is lower than 1.4:1,        preferably lower than 1:1, more preferably lower than 0.9:1.

An alternative preferred embodiment according to the present inventionis directed to a UV-Vis radiation hybrid curable security ink (i.e. anink comprising both cationically curable monomers and radically curablemonomers/oligomers) for producing a security feature exhibiting a bluecolor upon viewing in transmitted light and a metallic yellow color uponviewing in incident light, wherein said ink comprises:

-   -   a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets        having a mean diameter in the range of 50 to 150 nm with a        standard deviation of less than 60%, a mean thickness in the        range of 5 to 30 nm with a standard deviation of less than 50%,        and a mean aspect ratio higher than 2.0, wherein the mean        diameter is determined by transmission electron microscopy and        the mean thickness is determined by transmission electron        microscopy, and        wherein the silver nanoplatelets bear a surface stabilizing        agent of general formula (I)

-   -   wherein    -   the residue R^(A) is a C₂-C₄alkyl group substituted with a        hydroxy group;    -   the residue R^(B) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group; and    -   Cat⁺ is an ammonium cation of general formula ⁺NH₂R^(C)R^(D),    -   wherein the residue R^(C) is a C₂-C₄alkyl group substituted with        a hydroxy group; and    -   the residue R^(D) is selected from a C₁-C₄alkyl group, and a        C₂-C₄alkyl group substituted with a hydroxy group;    -   b) b-3) from about 45 wt-% to about 80 wt-% of a mixture of a        cycloaliphatic epoxide and one or more radically curable        monomers and/or oligomers; or        -   b-4) from about 45 wt-% to about 80 wt-%, preferably from            about 45 wt-% to about 65 wt-%, of a mixture of a            cycloaliphatic epoxide, one or more cationically curable            monomers and one or more radically curable monomers and/or            oligomers;    -   c) one or more cationic photoinitiators;    -   d) a perfluoropolyether surfactant functionalized with one or        more functional groups selected from the group consisting of:        hydroxyl, acrylate, methacrylate, and trialkoxysilyl;    -   e) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride        copolymer containing at least 60 wt-% of vinyl chloride;    -   g) one or more free radical photoinitiators; and optionally    -   f) up to about 25 wt-% of an organic solvent;        the weight percents being based on the total weight of the        UV-Vis radiation hybrid curable security ink. If the hybrid ink        claimed herein contains b-3) from about 45 wt-% to about 80 wt-%        of a mixture of a cycloaliphatic epoxide and one or more        radically curable monomers and/or oligomers, the ratio between        the total weight percent (wt-%) of the one or more radically        curable monomers and/or oligomers and the weight percent (wt-%)        of the cycloaliphatic epoxide is preferably lower than 1.6:1,        more preferably lower than 1:1, and even more preferably lower        than 0.5:1. If the hybrid ink claimed herein contains b-4) from        about 45 wt-% to about 80 wt-% of a mixture of a cycloaliphatic        epoxide, one or more cationically curable monomers and one or        more radically curable monomers and/or oligomers, the ratio        between the total weight percent (wt-%) of the one or more        radically curable monomers and/or oligomers and the sum of the        weight percent (wt-%) of the cycloaliphatic epoxide and of the        total weight percent (wt-%) of the one or more cationically        curable monomers is preferably lower than 1.6:1, more preferably        lower than 1:1, and even more preferably lower than 0.5:1, and        the ratio between the weight percent (wt-%) of the one or more        cationically curable monomers and the weight percent (wt-%) of        the cycloaliphatic epoxide is preferably lower than 1.4:1, more        preferably lower than 1:1 and even more preferably lower than        0.9:1.

Advantageously, the UV-Vis radiation cationically curable security inkclaimed herein and the UV-Vis radiation hybrid curable security inkclaimed herein provide security features with improved mechanicalresistance properties compared to the security features known in theart, which are obtained from UV radically curable inks or solvent-basedinks, and particularly from UV radically curable inks or solvent-basedinks containing high concentrations of silver nanoplatelets.

As well known to the skilled person, a cycloaliphatic epoxide is acationically curable monomer containing at least a substituted orunsubstituted epoxycyclohexyl residue:

Preferably, the cycloaliphatic epoxide described herein comprises atleast one cyclohexane ring, and at least two epoxide groups. Morepreferably, the cycloaliphatic epoxide is a compound of general formula(V):

wherein -L- represents a single bond or a divalent group comprising oneor more atoms. The cycloaliphatic epoxide of general formula (V) isoptionally substituted by one or more linear or branched alkyl radicalscontaining from one to ten carbon atoms (such as methyl, ethyl,n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl, hexyl, octyl,and decyl), and preferably containing from one to three carbon atoms(such as methyl, ethyl, n-propyl, and i-propyl).

In the general formula (V), the divalent group -L- may be a straight- orbranched-chain alkylene group comprising from one to eighteen carbonatoms. Examples of said straight- or branched-chain alkylene groupinclude without limitation methylene group, methylmethylene group,dimethylmethylene group, ethylene group, propylene group, andtrimethylene group.

In the general formula (V), the divalent group -L- may be a divalentalicyclic hydrocarbon group or cycloalkydene group such as1,2-cyclopentylene group, 1,3-cyclopentylene group, cyclopentylidenegroup, 1,2-cyclohexylene group, 1,3-cyclohexylene group,1,4-cyclohexylene group, and cyclohexylidene group.

In the general formula (V), -L- may be a divalent group comprising oneor more oxygen-containing linkage groups, wherein said oxygen-containinglinkage groups are selected from the group consisting of —C(═O)—,—OC(═O)O—, —C(═O)O—, and —O—. Preferably, the cycloaliphatic epoxide isa cycloaliphatic epoxide of general formula (V), wherein -L- is adivalent group comprising one or more oxygen-containing linkage groups,wherein said oxygen-containing linkage groups are selected from thegroup consisting of —C(═O)—, —OC(═O)O—, —C(═O)O—, and —O—, and morepreferably a cycloaliphatic epoxide of general formula (V-a), (V-b), or(V-c), as defined below:

wherein

L¹ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);

L² can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl); and

I1 and I2 are independently of each other integers comprised between 0and 9, preferably comprised between 0 and 3;

wherein

L¹ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);

L² can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl); and

I1 and I2 are independently of each other integers comprised between 0and 9, preferably comprised between 0 and 3;

-L³- is a single bond or a linear or branched divalent hydrocarbon groupcontaining from one to ten carbon atoms, and preferably containing fromthree to eight carbon atoms, such as alkylene groups includingtrimethylene, tetramethylene, hexamethylene, and 2-ethylhexylene, andcycloalkylene groups such as 1,2-cyclohexylene group, 1,3-cyclohexylenegroup, and 1,4-cyclohexylene group, and cyclohexylidene group;

wherein

L¹ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to three carbon atoms, suchas methyl, ethyl, n-propyl, and i-propyl;

L² can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to three carbon atoms, suchas methyl, ethyl, n-propyl, and i-propyl; and

I1 and I2 are independently of each other integers comprised between 0and 9, preferably comprised between 0 and 3.

Preferred cycloaliphatic epoxides of general formula (V-a) include, butare not limited to:3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate,3,4-epoxy-6-methyl-cyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate,3,4-epoxy-2-methyl-cyclohexylmethyl-3,4-epoxy-2-methyl-cyclohexanecarboxylate,and3,4-epoxy-4-methyl-cyclohexylmethyl-3,4-epoxy-4-methylcyclohexanecarboxylate.

Preferred cycloaliphatic epoxides of general formula (V-b) include, butare not limited to: bis(3,4-epoxycyclohexylmethyl)adipate,bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate,bis(3,4-epoxycyclohexylmethyl)oxalate,bis(3,4-epoxycyclohexylmethyl)pimelate, andbis(3,4-epoxycyclohexylmethyl)sebacate.

A preferred cycloaliphatic epoxide of general formula (V-c) is2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-meta-dioxane.

Further cycloaliphatic epoxides include a cycloaliphatic epoxide ofgeneral formula (VI-a) and a cycloaliphatic epoxide of general formula(VI-b), which are optionally substituted by one or more linear orbranched alkyl groups containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl).

The cycloaliphatic epoxides described herein may be hydroxy modified or(meth)acrylate modified. Examples are commercially available under thename Cyclomer A400 (CAS: 64630-63-3) and Cyclomer M100 (CAS number:82428-30-6) by Daicel Corp., or TTA 15 and TTA16 46 byTetraChem/Jiangsu.

The one or more cationically curable monomers described herein areselected from the group consisting of: vinyl ethers, propenyl ethers,cyclic ethers other than a cycloaliphatic epoxide, lactones, cyclicthioethers, vinyl thioethers, propenyl thioethers, hydroxyl-containingcompounds, and mixtures thereof, preferably from the group consistingof: vinyl ethers, cyclic ethers other than a cycloaliphatic epoxide, andmixtures thereof. Cyclic ethers other than a cycloaliphatic epoxideinclude epoxides other than a cycloaliphatic epoxide, oxetanes andtetrahydrofuranes. Preferably, the ratio between the total weightpercent (wt-%) of the one or more cationically curable monomers and theweight percent (wt-%) of the cycloaliphatic epoxide is lower than 1.4:1,more preferably lower than 1:1, most preferably lower than 0.9:1, andespecially preferably lower than 0.8:1.

Vinyl ethers are known in the art to accelerate curing and reducetackiness, thus limiting the risk of blocking and set-off when theprinted sheets are put in stacks just after printing and curing. Theyalso improve the physical and chemical resistance of the printedsecurity element and enhance the flexibility of the printed and curedink layer and its adhesion to the substrate, which is particularlyadvantageous for printing on plastic and polymer substrates. Vinylethers also help reducing the viscosity of the ink while stronglyco-polymerizing with the ink vehicle. Examples of preferred vinyl ethersto be used in the security ink claimed herein include methyl vinylether, ethyl vinyl ether, n-propyl vinyl ether, n-butyl vinyl ether,iso-butyl vinyl ether, ethylhexyl vinyl ether, octadecyl vinyl ether,dodecyl vinyl ether, isopropyl vinyl ether, tert-butyl vinyl ether,tert-amyl vinyl ether, cyclohexyl vinyl ether, cyclohexanedimethanolmonovinyl ether, cyclohexanedimethanol divinyl ether, 4-(vinyloxymethyl)cyclohexylmethyl benzoate, phenyl vinyl ether, methylphenyl vinylether, methoxyphenyl vinyl ether, 2-chloroethyl vinyl ether,2-hydroxyethyl vinyl ether, 4-hydroxybutyl vinyl ether, 1,6-hexanediolmonovinyl ether, ethylene glycol divinyl ether, ethylene glycolmonovinyl ether, 1, 4-butanediol divinyl ether, 1,6-hexanediol divinylether, 4-(vinyloxy)butyl benzoate, bis[4-(vinyl oxy)butyl]adipate,bis[4-(vinyloxy)butyl]succinate,bis[4-(vinyloxymethyl)cyclohexylmethyl]glutarate, 4-(vinyloxy)butylstearate, trimethylolpropane trivinyl ether, propenyl ether of propylenecarbonate, diethylene glycol monovinyl ether, diethylene glycol divinylether, ethylene glycol butylvinyl ether, dipropylene glycol divinylether, triethylene glycol divinyl ether, triethylene glycol methyl vinylether, triethylene glycol monobutyl vinylether, tetraethylene glycoldivinyl ether, poly(tetrahydrofuran) divinyl ether,polyethyleneglycol-520 methyl vinyl ether, pluriol-E200 divinyl ether,tris[4-(vinyloxy)butyl]trimellitate, 1,4-bis(2-vinyloxyethoxy)benzene,2,2-bis(4-vinyloxyethoxyphenyl)propane,bis[4-(vinyloxy)methyl]cyclohexyl]methyl] terephthalate,bis[4-(vinyloxy)methyl]cyclohexyl]methyl] isophthalate. Suitable vinylethers are commercially sold by BASF under the designation EVE, IBVE,DDVE, ODVE, BDDVE, DVE-2, DVE-3, CHVE, CHDM-di, HBVE. The one or morevinyl ethers described herein may be hydroxy modified or (meth)acrylatemodified (for example: VEEA, 2-(2-vinyloxyethoxy)ethyl acrylate fromNippon Shokubai (CAS: 86273-46-3)).

Oxetanes are known in the art to accelerate curing and reduce tackiness,thus limiting the risk of blocking and set-off when the printed sheetsare put in stacks just after printing and curing. They also helpreducing the viscosity of the ink while strongly co-polymerizing withthe ink vehicle. Preferred examples of oxetanes include trimethyleneoxide, 3,3-dimethyloxetane, trimethylolpropane oxetane,3-ethyl-3-hydroxymethyl oxetane, 3-ethyl-3-[(2-ethylhexyloxy)methyl]oxetane, 3,3-dicyclomethyl oxetane, 3-ethyl-3-phenoxymethyloxetane, bis ([1-ethyl(3-oxetanyl)]methyl) ether, 1,4-bis[3-ethyl-3-oxetanyl methoxy)methyl]benzene,3,3-dimethyl-2(p-methoxy-phenyl)-oxetane, 3-ethyl-[(tri-ethoxysilylpropoxy)methyl]oxetane,4,4-bis(3-ethyl-3-oxetanyl)methoxymethyl]biphenyl and3,3-dimethyl-2(p-methoxy-phenyl) oxetane. The one or more oxetanesdescribed herein may be hydroxy modified or (meth)acrylate modified (forexample: UVi-Cure S170 from Lambson (CAS: 37674-57-0)).

The use of epoxides in the UV-Vis radiation curable ink aids inaccelerating curing and reducing tackiness, as well as in reducing theviscosity of the ink while strongly co-polymerizing with the inkvehicle. Preferred examples of an epoxide other than a cycloaliphaticepoxide as described herein include, but are not limited to, cyclohexanedimethanol diglycidylether, poly(ethyleneglycol) diglycidyl ether,poly(propyleneglycol) diglycidyl ether, butanediol diglycidyl ether,hexanediol diglycidyl ether, bisphenol-A diglycidyl ether,neopentylglycol diglycidylether, trimethylolpropane triglycidyl ether,glycerol triglycidyl ether, pentaerythritol tetraglycidyl ether, butylglycidyl ether, p-tert-butyl phenyl glycidyl ether, hexadecyl glycidylether, 2-ethyl-hexyl glycidyl ether, octyl glycidyl ether, decylglycidyl ether, dodecyl glycidyl ether, tetradecyl glycidyl ether,C₁₂/C₁₄-alkyl glycidyl ether, C₁₃/C₁₅-alkyl glycidyl ether and mixturesthereof. Suitable epoxides other than a cycloaliphatic epoxide arecommercially sold by EMS Griltech under the trademark Grilonit® (e.g.Grilonit® V51-63 or RV 1806).

The radically curable monomer described herein is selected from thegroup consisting of mono(meth)acrylates, di(meth)acrylates,tri(meth)acrylates, tetra(meth)acrylates, and mixtures thereof,preferably from the group consisting of tri(meth)acrylates,tetra(meth)acrylates, and mixtures thereof. The term “(meth)acrylate” inthe context of the present invention refers to the acrylate as well asthe corresponding methacrylate.

Preferred examples of mono(meth)acrylates include 2(2-ethoxyethoxy)ethyl(meth)acrylate, 2-phenoxyethyl (meth)acrylate, C₁₂/C₁₄ alkyl(meth)acrylate, C₁₆/C₁₈ alkyl (meth)acrylate, caprolactone(meth)acrylate, cyclic trimethylolpropane formal (meth)acrylate,nonylphenol (meth)acrylate, isobornyl (meth)acrylate, isodecyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,octyldecyl (meth)acrylate, tridecyl (meth)acrylate, methoxypoly(ethylene glycol) (meth)acrylate, polypropylene glycol(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 1,3-butylene glycoldi(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, 3-methyl-1,5-pentanedioldi(meth)acrylate, alkoxylateddi(meth)acrylate, esterdiol di(meth)acrylate as well as mixturesthereof.

Preferred examples of di(meth)acrylates include bisphenol Adi(meth)acrylates, alkoxylated (such as for example ethoxylated andpropoxylated) bisphenol A di(meth)acrylate, bisphenol A diglycidyl etherdi(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, dipropyleneglycol di(meth)acrylate,tripropylene glycol di(meth)acrylate, polyethylene glycoldi(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, as well as mixtures thereof.

Preferred examples of tri(meth)acrylates include trimethylolpropanetri(meth)acrylates, alkoxylated (such as for example ethoxylated andpropoxylated) trimethylolpropane tri(meth)acrylates, alkoxylated (suchas for example ethoxylated and propoxylated) glyceroltri(meth)acrylates, pentaerythritol tri(meth)acrylates, alkoxylatedpentaerythritol tri(meth)acrylates, alkoxylated (such as for exampleethoxylated and propoxylated) pentaerythritol tri(meth)acrylates, aswell as mixtures thereof.

Preferred examples of tetra(meth)acrylates include ditrimethylolpropanetetra(meth)acrylates, pentaerythritol tetra(meth)acrylates, alkoxylated(such as for example ethoxylated and propoxylated) pentaerythritoltetra(meth)acrylates and mixtures thereof, preferably selected from thegroup consisting of ditrimethylolpropane tetra(meth)acrylates,alkoxylated pentaerythritol tetra(meth)acrylates, as well as mixturesthereof.

As used herein, the term “radically curable oligomer” refers to aradically curable (meth)acrylate oligomer that may be branched oressentially linear, and may have terminal and/or pendant (meth)acrylatefunctional group(s). Preferably, the radically curable oligomer isselected from the group consisting of (meth)acrylic oligomers, urethane(meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyetherbased (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, andmixtures thereof, more preferably selected from the group consisting ofpolyester (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, andmixtures thereof.

Suitable examples of epoxy (meth)acrylate oligomer include withoutlimitation aliphatic epoxy (meth)acrylate oligomers, in particularmono(meth)acrylates, di(meth)acrylates and tri(meth)acrylates, andaromatic epoxy (meth)acrylate oligomers. Suitable examples of aromaticepoxy (meth)acrylate oligomers include bisphenol-A (meth)acrylateoligomers such as bisphenol-A mono(meth)acrylates, bisphenol-Adi(meth)acrylates and bisphenol-A tri(meth)acrylates as well asalkoxylated (such as for example ethoxylated and propoxylated)bisphenol-A (meth)acrylate oligomers such as for example alkoxylatedbisphenol-A mono(meth)acrylates, alkoxylated bisphenol-Adi(meth)acrylates and alkoxylated bisphenol-A tri(meth)acrylates,preferably alkoxylated bisphenol-A di(meth)acrylates.

The security ink claimed herein contains c) one or more cationicphotoinitiators. Preferably, the amount of the one or more cationicphotoinitiators in the UV-Vis radiation cationically curable securityink claimed herein (i.e. the ink containing exclusively cationicallycurable monomers and no radically curable monomers) is from about 1 wt-%to about 10 wt-%, preferably from about 1.1 wt-% to about 8 wt-%, morepreferably from about 1.1 wt-% to about 6 wt-%, wherein the weightpercent is based on the total weight of the UV-Vis radiationcationically curable ink. Preferably, the amount of the one or morecationic photoinitiators in the UV-Vis radiation hybrid curable securityink claimed herein (i.e. the ink containing both cationically curablemonomers and radically curable monomers) is from 1 wt-% to about 6 wt-%,wherein the weight percent is based on the total weight of the UV-Visradiation cationically curable ink.

The one or more cationic photoinitiators described herein (also referredin the art as photo-acid generators) are onium salts preferably selectedfrom the group consisting of azonium salts, oxonium salts, iodoniumsalts, sulfonium salts and mixtures thereof, more preferably selectedfrom the group consisting of oxonium salts, iodonium salts, sulfoniumsalts, and mixtures thereof, and even more preferably selected from thegroup consisting of sulfonium salts, iodonium salts, and mixturesthereof.

The iodonium salts described herein have a cationic moiety and ananionic moiety, wherein the anionic moiety is preferably BF₄ ⁻, B(C₆F₅)₄⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻ or CF₃SO₃ ⁻, more preferably SbF₆ ⁻ and whereinthe cationic moiety is preferably an aromatic iodonium ion, morepreferably a iodonium ion comprising two aryl groups, wherein the twoaryl groups may be independently substituted by one or more alkyl groups(such as for example methyl, ethyl, isobutyl, tertbutyl, etc.), one ormore alkoxy groups, one or more nitro groups, one or more halogencontaining groups, one or more hydroxy groups or a combination thereof,preferably by one or more alkyl groups. Particularly suitable iodoniumsalts for the present invention are commercially available known underthe name DEUTERON UV 1240, DEUTERON UV 1242, DEUTERON UV 2257, DEUTERONUV 1250, and DEUTERON UV 3100, all available from DEUTERON, OMNICAT 250,OMNICAT 440, and OMNICAT 445, all available from IGM Resins, SpeedCure937, SpeedCure 938 and SpeedCure 939, all available from Lambson.

The sulfonium salts described herein have a cationic moiety and ananionic moiety, wherein the anionic moiety is preferably BF₄ ⁻, B(C₆F₅)₄⁻, PF₆ ⁻, (PF_(6-h)(C_(j)F_(2j-1))_(h))— (where h is an integer from 1to 5, and j is an integer from 1 to 4), AsF₆ ⁻, SbF₆ ⁻, CF₃SO₃ ⁻,perfluoroalkyl sulfonate or pentafluoro-hydroxyantimonate, morepreferably SbF₆ ⁻ and wherein the cationic moiety is preferably anaromatic sulfonium ion, more preferably a sulfonium ion comprising twoor more aryl groups, wherein the two or more aryl groups may beindependently substituted by one or more alkyl groups (such as forexample methyl, ethyl, isobutyl, tertbutyl, etc.) one or more alkoxygroups, one or more aryloxyl groups, one or more halogen containinggroups, one or more hydroxy groups or a combination thereof. Suitableexamples of sulfonium ions comprising two or more aryl groups includewithout limitation triarylsulfonium ions, diphenyl[4-(phenylthio)phenyl]sulfonium ion, bis[4-(diphenylsulfonio)phenyl]sulfonium ion,triphenylsulfonium ions, and tris[4-(4-acetylphenyl)sulfanylphenyl]sulfonium ion. Particularly suitable examples of sulfonium salts for thepresent invention are commercially available under the name SpeedCure976, SpeedCure 976D, SpeedCure 976S and SpeedCure 992, all availablefrom Lambson, ESACURE 1187, OMNICAT 270, OMNICAT 320, OMNICAT 432 andOMNICAT 550, all available from IGM Resins, DoubleCure 1176, DoubleCure1190 and DoubleCure 1172, all available from DoubleBond.

The oxonium salts described herein have a cationic moiety and an anionicmoiety, wherein the anionic moiety is preferably BF₄ ⁻, B(C₆F₅)₄ ⁻, PF₆⁻, AsF₆ ⁻, SbF₆ ⁻ or CF₃SO₃ ⁻, more preferably BF₄ ⁻ and wherein thecationic moiety is preferably an aromatic oxonium ion, more preferably apyrylium ion preferably substituted by one or more aryl groups, whereinthe one or more aryl groups may be independently of each othersubstituted by one or more alkyl groups (such as for example methyl,ethyl, isobutyl, tertbutyl, etc.), one or more alkoxy groups, one ormore nitro groups, one or more halogen groups, one or more hydroxygroups or a combination thereof. A particularly suitable oxonium saltfor the present invention is 2,4,6-triphenylpyrylium tetrafluoroborate.

Other examples of useful cationic photoinitiators can be found instandard textbooks such as “Chemistry & Technology of UV & EBFormulation for Coatings, Inks & Paints”, Volume III, “Photoinitiatorsfor Free Radical Cationic and Anionic Polymerization”, 2nd edition, byJ. V. Crivello & K. Dietliker, edited by G. Bradley and published in1998 by John Wiley & Sons in association with SITA Technology Limited.

Moreover, the hybrid security ink claimed herein contains g) one or morefree radical photoinitiators. Preferably, the amount of the one or morefree radical photoinitiators in the UV-Vis radiation hydrid curable inkdescribed herein is from about 1 wt-% to about 6 wt-%, the percent beingbased on the total weight of the UV-Vis radiation hydrid curable ink.

The one or more free radical photoinitiators as used herein arepreferably selected form the group consisting of hydroxyketones (e.g.alpha-hydroxyketones), alkoxyketones (e.g. alpha-alkoxyketones),acetophenones, benzophenones, ketosulfones, benzyl ketals, benzoinethers, phosphine oxides, phenylglyoxylates, thioxanthones, and mixturesthereof, more preferably selected form the group consisting of phosphineoxides, hydroxyketones, thioxanthones and mixtures thereof.

Suitable alpha-hydroxyketones include without limitation(1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one),1-hydroxycyclohexyl phenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-2-methyl-1-(4-tert-butyl)phenylpropan-1-one,2-hydroxy-1-[4-[[4-(2-hydroxy-2-methylpropanoyl)phenyl]methyl]phenyl]-2-methylpropan-1-one,2-hydroxy-1-[4-[4-(2-hydroxy-2-methylpropanoyl)phenoxy]phenyl]-2-methylpropan-1-one,and oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone].

Suitable acetophenones include without limitation2,2-diethoxyacetophenone, and 2-methoxy-2-phenylacetophenone.

Suitable benzophenones include without limitation benzophenone,polymeric benzophenone derivatives, 2-methylbenzophenone,3-methylbenzophenone, 4-methylbenzophenone, 2,4,6-trimethylbenzophenone,3,3′-dimethyl-4-methoxybenzophenone, 4-phenylbenzophenone,4-chlorobenzophenone, methyl-2-benzoylbenzoate,4-(4-methylphenylthio)benzophenone, 4-hydroxybenzophenone laurate, and amixture of 50% benzophenone and 50% 1-hydroxycyclohexyl phenyl ketone.

Suitable ketosulfones include without limitation1-[4-(4-benzoylphenylsulfanyl)phenyl]-2-methyl-2-(4-methylphenylsulfonyl)propan-1-one.

Suitable benzyl ketals include without limitation2,2-dimethoxy-2-phenylacetophenone.

Suitable benzoin ethers include without limitation2-ethoxy-1,2-diphenylethanone, 2-isopropoxy-1,2-diphenylethanone,2-isobutoxy-1,2-diphenylethanone, 2-butoxy-1,2-diphenylethanone,2,2-dimethoxy-1,2-diphenylethanone, and 2,2-diethoxyacetophenone.

Suitable phosphine oxides include without limitation2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethylphenyl(2,4,6-trimethylbenzoyl)phenylphosphinate,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide,substituted acyl-phosphine oxides, a mixture ofdiphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and2-hydroxy-2-methylpropiophenone, a mixture ofphenylbis(2,4,6-trimethylbenzoyl)phosphine oxide and2-hydroxy-2-methylpropiophenone, a mixture ofethyl(2,4,6-trimethylbenzoyl)phenylphosphinate and2-hydroxy-2-methylpropiophenone, and a mixture ofphenylbis(2,4,6-trimethylbenzoyl)phosphine oxide and ethylphenyl(2,4,6-trimethylbenzoyl)phenylphosphinate.

Suitable thioxanthones include without limitation 2-methyl thioxanthone,2,4-diethylthioxanthone, 2-isopropylthioxanthone,1-chloro-4-propoxythioxanthone, and polymeric thioxanthone derivatives.

Suitable phenylglyoxylates include without limitation methylbenzoylformate, 2-[2-oxo-2-phenyl-acetoxy-ethoxy]ethyl2-oxo-2-phenylacetate, and a mixture of2-[2-oxo-2-phenyl-acetoxy-ethoxy]ethyl 2-oxo-2-phenylacetate andoxy-phenyl-acetic acid 2-[2-hydroxy-ethoxy]-ethyl ester.

Preferably, the one or more free radical photoinitiators are phosphineoxides as described herein, and more preferably a mixture ofphenylbis(2,4,6-trimethylbenzoyl)phosphine oxide and ethylphenyl(2,4,6-trimethylbenzoyl)phenylphosphinate.

The UV-Vis radiation curable security ink described herein contains d) aperfluoropolyether surfactant functionalized with one or more functionalgroups selected from the group consisting of: hydroxyl, acrylate,methacrylate, and trialkoxysilyl, preferably two or more functionalgroups selected from the group consisting of: hydroxyl, acrylate,methacrylate, and trialkoxysilyl. Surprisingly, it has been found thatthe use of a perfluoropolyether functionalized with one or more,preferably two or more, functional groups selected from the groupconsisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl, assurfactant in the UV-Vis radiation curable ink described herein isessential for producing security features exhibiting a metallic yellowcolor upon viewing in incident light. As attested for example by Table3c and Table 4c, only UV-Vis radiation curable inks containing aperfluoropolyether surfactant functionalized with one or more functionalgroups selected from the group consisting of: hydroxyl, acrylate,methacrylate, and trialkoxysilyl provide security features exhibiting ametallic yellow color upon viewing in incident light. The securityfeatures produced with a UV-Vis radiation curable ink lacking asurfactant (for e.g.: ink C8), or comprising either a perfluoropolyethersurfactant lacking the functional group (for e.g. inks C1), or asurfactant lacking the perfluoropolyether backbone (for e.g. inks C2-C7)show a brown to dark brown color in reflection, which is noteye-catching for the layperson, and therefore not suitable for adichroic security feature for securing a value document.

The perfluoropolyether surfactant functionalized with one or morefunctional groups selected from the group consisting of: hydroxyl,acrylate, methacrylate, and trialkoxysilyl, comprises aperfluoropolyether backbone and one or more, preferably two or more,terminal functional groups selected from the group consisting of:hydroxyl, acrylate, methacrylate and trialkoxysilyl is characterized byan average molecular weight (Mn) below about 2000 [g/mol]. As usedherein, a perfluoropolyether backbone denotes a residue of aperfluoropolyether polymer comprising randomly distributed recurringunits selected from perfluoromethyleneoxy (—CF₂O—) andperfluoroethyleneoxy (—CF₂—CF₂O—). The perfluoropolyether residue isconnected to the terminal functional group directly or via a spacerselected from methylene(oxyethylene),1,1-difluoroethylene-(oxyethylene), methylene-di(oxyethylene),1,1-difluoroethylene-di(oxyethylene), methylene-tri(oxyethylene),1,1-difluoroethylene-tri(oxyethylene), methylene-tetra(oxyethylene),1,1-difluoroethylene-tetra(oxyethylene), methylene-penta(oxyethylene),1,1-difluoroethylene-penta(oxyethylene), and a linear or branchedhydrocarbon group, optionally fluorinated at the carbon atom connectingthe spacer to the perfluoropolyether residue, containing one or moreurethane groups, or one or more amide groups, and optionally one or morecyclic moieties, including saturated cyclic moieties (such ascyclohexylene) and aromatic cyclic moieties (such as phenylene).Preferably, the perfluoropolyether surfactant is functionalized with oneor more hydroxyl functional groups.

In a further preferred embodiment, the perfluoropolyether surfactantfunctionalized with one or more functional groups selected from thegroup consisting of: hydroxyl, acrylate, methacrylate, andtrialkoxysilyl is a compound of general formula (VII) having an averagemolecular weight from about 1200 [g/mol] to about 2000 [g/mol]

whereinf and e are independently of each other an integer selected from 1, 2and 3;FG¹ and FG² are terminal functional groups selected independently ofeach other from the group consisting of: —OH, —OC(O)CH═CH₂,—OC(O)C(CH₃)═CH₂, and —Si(OR²⁰)₃;R²⁰ is a C₁-C₄alkyl group;—S¹— represents a single bond or a spacer selected from:

wherein-J¹- is selected from

whereinj¹ is an integer comprised between 1 and 12, preferably between 4 and10;L⁵ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);L⁶ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);I⁵ and I⁶ are independently of each other integers comprised between 0and 4, preferably comprised between 0 and 1; and-J³- is selected from —O—, —CH₂—, —CH(CH₃)—, and —C(CH₃)₂—;-J²- is selected from

a is an integer comprised between 1 and 6, preferably between 1 and 3;andb is an integer comprised between 1 and 6, preferably between 2 and 4;—S²— represents a single bond or a spacer selected from

wherein-J⁴- is selected from

whereinj⁴ is an integer comprised between 1 and 12, preferably between 4 and10;L⁷ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);L⁸ can be the same, or different in each occurrence and is a linear orbranched alkyl radical containing from one to ten carbon atoms (such asmethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl,hexyl, octyl, and decyl), and preferably containing from one to threecarbon atoms (such as methyl, ethyl, n-propyl, and i-propyl);I⁷ and I⁸ are independently of each other integers comprised between 0and 4, preferably comprised between 0 and 1; and-J⁶- is selected from —O—, —CH₂—, —CH(CH₃)—, and —C(CH₃)₂—;-J⁵- is selected from

wherein r is an integer comprised between 1 and 6, preferably between 1and 3; andw is an integer comprised between 1 and 6, preferably between 2 and 4;and wherein s and t are integers chosen so that the average molecularweight of the compound of general formula (VII) is from about 1200[g/mol] to about 2000 [g/mol].

Preferably, in general formula (VII), FG¹ and FG² representindependently of each other —OC(O)CH═CH₂, or —OC(O)CCH₃)═CH₂;

—S¹— represents

wherein b has the meaning defined herein; and—S²— represents

wherein w has the meaning defined herein.

Also preferably, in general formula (VII), FG¹ and FG² represent —OH;

—S¹— represents a single bond or

wherein a has the meaning defined herein;—S²— represents a single bond or

wherein r has the meaning defined herein; and the sum of o and r iscomprised between 3 and 9.

Also preferably, in general formula (VII), FG¹ and FG² represent—Si(OR²⁰)₃;

R²⁰ is a C₁-C₄alkyl group, preferably an ethyl group;—S¹— represents

wherein b has the meaning defined herein; and—S²— represents

wherein w has the meaning defined herein. Thus, a preferredperfluoropolyether surfactant is a compound of general formula (VII-a)

whereinb and w are integers comprised between 1 and 6, preferably between 2 and4;s is a integer of between 2 and 6; andq is an integer of between 2 and 4.

Particularly suitable examples of perfluoropolyether surfactantfunctionalized with one or more functional groups selected from thegroup consisting of: hydroxyl, acrylate, methacrylate and trialkoxysilylfor the present invention are commercially available under the nameFluorolink® E10H, Fluorolink® MD700, Fluorolink® AD1700, Fluorolink®E-series, and Fluorolink® S10 from Solvay.

The UV-Vis radiation curable security ink claimed herein contains e)from about 3 wt-% to about 12 wt-% of a polyvinyl chloride copolymercontaining at least 60 wt-% of vinyl chloride, preferably at least 63wt-% of vinyl chloride. UV-Vis radiation curable security inkscontaining no polyvinyl chloride copolymer provide security featureswith non-attractive colors, such as brown or dark brown, and low chromavalue C* upon viewing in incident light and consequently, are notsuitable to be used for the production of security feature showing ametallic yellow color in incident light.

It is preferred that the polyvinyl chloride copolymer contains at themost 90 wt-% of vinyl chloride.

Preferably, the polyvinyl chloride copolymer containing at least 60 wt-%of vinyl chloride is present in the security ink claimed herein in anamount from about 4.9 wt-% to about 11.6 wt-%, and most preferably fromabout 6 wt-% to about 8.6 wt-%, wherein the weight percents are based onthe total weight of the UV-Vis radiation curable ink.

Preferably, the polyvinyl chloride copolymer is selected from the groupconsisting of vinyl chloride-vinyl acetate copolymer, vinylchloride-hydroxyalkylacrylate copolymer, such as vinylchloride-2-hydroxypropyl acrylate copolymer, and vinylchloride-hydroxyalkylacrylate-Z-alkylenedioic acid, dialkyl estercopolymer, such as vinyl chloride-2-hydroxypropyl acrylate-2-butenedioicacid (Z)-, dibutyl ester copolymer. The polyvinyl chloride copolymer haspreferably an average molecular weight of between 3*10⁴ g/mol and about8*10⁴ g/mol as determined by size exclusion chromatography usingpolystyrene as standard and tetrahydrofuran as solvent. Particularlysuitable examples of polyvinyl chloride copolymer for the presentinvention are commercially available under the name Vinnol® H14/36,Vinnol® E22/48A, Vinnol® E 15/40 A and Vinnol® H 40/50 from Wacker.

The UV-Vis radiation curable security inks claimed herein may contain f)up to about 25 wt-% of an organic solvent, the weight percent beingbased on the total weight of the UV-Vis radiation curable ink. Thesolvent has a boiling point higher than 100° C. Suitable organicsolvents to be used in the UV-Vis radiation curable inks describedherein include without limitation: ethyl-3-ethoxypropionate,2-methoxy-1-methylethyl acetate, propylene glycol mono methyl ether,cyclopentanone, cyclohexanone, n-butanol, cyclohexanol, ethylenecarbonate, propylene carbonate, butylene carbonate, and mixturesthereof.

In a preferred embodiment according to the present invention, the UV-Visradiation curable security ink is solvent-free. The use of asolvent-free ink in an industrial printing process of value documents isof high interest because it prevents emission of volatile organiccomponents, which typically have a negative impact on the environmentand are harmful for human health.

The UV-Vis radiation curable security ink claimed herein may furthercomprise one or more photosensitizers in conjunction with the one ormore photoinitiators described herein in order to achieve efficientcuring. Suitable examples of photosensitizers are known to those skilledin the art (e.g. in Industrial Photoinitiators, W. A. Green, CRC Press,2010, Table 8.1 p 0.170). Preferred photosensitizers are those that areable to achieve efficient and fast curing with UV-LED light sources,such as thioxanthone derivatives, anthracene derivatives and naphthalenederivatives (such as 9,10-diethoxyanthracene sold as Anthracure UVS-1101and 9,10-dibutyloxyanthracene sold as Anthracure UVS-1331, both sold byKawasaki Kasei Chemicals Ltd) and titanocene derivatives (such asIrgacure 784 sold by BASF). Particularly preferred are thioxanthonederivatives, including without limitation isopropyl-thioxanthone (ITX),1-chloro-2-propoxy-thioxanthone (CPTX), 2-chloro-thioxanthone (CTX) and2,4-diethyl-thioxanthone (DETX), and mixtures thereof. Alternatively,thioxanthone photosensitizers may be used in an oligomeric or polymericform (such as Omnipol TX sold by IGM Resins, Genopol* TX-2 sold by Rahn,or Speedcure 7010 sold by Lambson). When present, the one or morephotosensitizers are preferably present in an amount from about 0.1 wt-%to about 2 wt-%, more preferably from about 0.2 wt-% to about 1 wt-%,the weight percent being based on the total weight of the UV-Visradiation curable ink.

The UV-Vis radiation curable ink claimed herein may further comprise oneor more antifoaming agents in an amount of less than about 2 wt-%,preferably of less than about 1 wt-%.

Another aspect according to the present invention is directed to aprocess for producing a security feature for securing a value document,wherein said security feature exhibits a blue color upon viewing intransmitted light and a metallic yellow color upon viewing in incidentlight, said process comprising the following steps:

-   -   A) printing, preferably by screen printing, rotogravure, or        flexography, the UV-Vis radiation curable security ink claimed        herein on a transparent or partially transparent region of a        substrate of a value document to provide an ink layer; and    -   B) UV-Vis curing the ink layer obtained at step B) to form the        security feature.

The inventive manufacturing process claimed herein enables access in asingle printing step to a security feature displaying a metallic yellowcolor in incident light and a blue color, especially an intense to veryintense blue color, in transmitted light. As used herein, the term“printing” refers to any printing process suitable for printing theUV-Vis radiation curable ink described herein on a substrate of a valuedocument. In particularly, the term “printing” refers to a printingprocess selected from the group consisting of: screen printing,rotogravure, flexography, pad printing, inkjet printing, and sprayprinting. Preferably, the UV-Vis radiation curable security ink isprinted on a transparent or partially transparent region of thesubstrate of the value document by screen printing, rotogravure orflexography, more preferably by screen printing.

As used herein, “a transparent or partially transparent region of asubstrate of a value document” refers to a region of the substrate ofthe value document, wherein said region is characterized by an averagetransmittance in the visible range of at least 50%, preferably of atleast 70%, more preferably of at least 90%. The transparent or partiallytransparent region of the substrate and the remaining region of thesubstrate may be made either of the same material, or of differentmaterials. Elimination of one or more layers in a multilayer structureor application of a transparent or partially transparent material to anaperture in a substrate made of a material, which is different from thetransparent or partially transparent material provides value documentssubstrates, wherein the transparent or partially transparent region ofthe substrate and the remaining region of the substrate are made ofdifferent materials.

Materials for value document substrates include without limitation,papers or other fibrous materials such as cellulose, paper-containingmaterials, plastics and polymers, composite materials, and mixtures orcombinations thereof. Typical paper, paper-like or other fibrousmaterials are made from a variety of fibers including without limitationabaca, cotton, linen, wood pulp, and blends thereof. As well known tothose skilled in the art, cotton and cotton/linen blends are preferredfor banknotes, while wood pulp is commonly used in non-banknote securitydocuments. Typical examples of plastics and polymers includepolystyrene, polycarbonate, polyolefins, such as polyethylene (PE) andpolypropylene (PP) including biaxially-oriented polypropylene (BOPP),polyamides (PA), polyesters such as poly(ethylene terephthalate) (PET),polyethylene terephthalate glycol-modified (PETG) includingpoly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate),poly(1,4-butylene terephthalate) (PBT), and poly(ethylene2,6-naphthoate) (PEN), and polyvinylchlorides (PVC). Typical examples ofcomposite materials include without limitation multilayer structures orlaminates of paper and at least one plastic or polymer material, such asthose described hereabove. Suitable materials for the transparent orpartially transparent region of the substrate include, but are notlimited to polystyrene, polycarbonate, polyolefins, such as polyethylene(PE) and polypropylene (PP) including biaxially-oriented polypropylene(BOPP), polyamides (PA), polyesters such as poly(ethylene terephthalate)(PET), polyethylene terephthalate glycol-modified (PETG) includingpoly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate),poly(1,4-butylene terephthalate) (PBT), and poly(ethylene2,6-naphthoate) (PEN), and polyvinylchlorides (PVC). The transparent orpartially transparent region of the substrate of the value document maycarry a primer layer on the top of which the UV-Vis radiation curableink is printed. The primer layer may be obtained by UV-Vis curing avarnish containing all the ingredients of the UV-Vis radiation curableink described herein, with the exception of the silver nanoplatelets.

At step B) of the inventive manufacturing process claimed herein, theink layer obtained at step A) is subjected to UV-Vis curing to form thesecurity feature. As used herein, the term “UV-Vis curing” refers toradiation-curing of the ink layer by photo-polymerization, under theinfluence of an irradiation having wavelength components in the UV or inthe UV and visible part of the electromagnetic spectrum (typically 100nm to 800 nm, preferably between 150 and 600 nm and more preferablybetween 200 and 400 nm). Cationically curable monomers are cured bycationic mechanisms consisting of the activation by UV-Vis light of oneor more photoinitiators, which liberate cationic species, such as acids,which in turn initiate the polymerization of the compound so as to forma cured binder. Radically curable monomers and oligomers are cured byfree radical mechanisms consisting of the activation by UV-Vis light ofone or more photoinitiators, which liberate free radicals which in turninitiate the polymerization process. Optionally, one or morephotosensitizers may also be present. Photosensitizers are activated byone or more of the wavelengths emitted by a UV-Vis light source andreach an excited state. The excited photosensitizer either transferenergy to the one or more photoinitiators (in free-radicalpolymerization) or an electron (in cationic polymerization). Eitherprocess in turn initiates the polymerization process.

Preferably, step B) comprises exposure of the ink layer obtained at stepA) to UV-Vis light emitted by a UV-Vis light source selected from thegroup consisting of: mercury lamps, preferably medium-pressure mercurylamps, UV-LED lamps, and sequences thereof. Typical sequences includethe use of one or more UV-LED lamps in a first step to partially curethe UV-Vis radiation composition and one or more medium-pressure mercurylamps in a second step. Mercury lamps advantageously emit on a widerange of wavelengths in the UV-A, UV-B and UV-C range. Accordingly,there is a large selection of photoinitiators or combinations ofphotoinitiator/photosensitizer having an absorption spectrum matching atleast one of the emission band of the mercury lamp. UV-LED have a morelimited range of wavelengths, such that only a limited selection ofphotoinitiators or combination of photoinitiator/photosensitizer isefficient enough at industrial printing speed. On the other hand,UV-LEDs are less costly, require less energy (in particular, they needmuch less demanding heat dissipation systems), are not prone to ozoneformation and have a much longer lifespan.

To provide the value document with soil resistance and/or to protect thesecurity feature against physical and chemical attacks from theenvironment, the manufacturing process claimed herein preferably furthercomprises steps C) and D) conducted after step B):

-   -   C) applying on the substrate, preferably by a printing process,        a curable protective varnish to form a varnish layer;    -   D) curing the varnish layer obtained at step C) so as to form a        protective coating.

Examples of suitable curable protective varnishes to be used at step C)and/or of methods of applying said curable protective varnishes on thesubstrate and of curing the varnish layer are described in theinternational patent application publication number WO2020234211A1, theinternational patent application publication number WO2013127715A2 andthe international patent application publication number WO2014067715A1.

Preferably, the value document is selected from banknotes, deeds,tickets, checks, vouchers, fiscal stamps, agreements, identity documentssuch as passports, identity cards, visas, driving licenses, bank cards,credit cards, transactions cards, access documents, and cards, entrancetickets, public transportation tickets, academic diploma, and academictitles. More preferably the value document is a banknote. The securityink claimed herein may be also used for producing a security featuredirectly on a value commercial good. The term “value commercial good”refers to packaging material, in particular for pharmaceutical,cosmetics, electronics or food industry that may be protected againstcounterfeiting and/or illegal reproduction in order to warrant thecontent of the packaging like for instance genuine drugs.

Examples

The present invention is now described in more details with reference tonon-limiting examples. The examples E1-E26 and comparative examplesC1-C8 below provide more details for the preparation the UV-Visradiation curable screen printing security inks described herein andoptical properties of security features obtained therefrom.

A. Analytical Methods

A-1. UV-Vis Spectroscopy

UV-Vis spectra of dispersions were recorded on Varian Cary 50 UV-Visiblespectrophotometer at such concentration of dispersions as to achieve theoptical density of 0.3 to 1.5 at 1 cm optical path.

A-2. TEM Analysis

TEM analysis of dispersions was performed on EM 910 instrument fromZEISS in bright field mode at an e-beam acceleration voltage of 100 kV.At least 2 representative images with scale in different magnificationwere recorded in order to characterize the dominant particle morphologyfor each sample.

The mean diameter of the silver nanoplatelets was determined bytransmission electron microscopy (TEM) using Fiji image analysissoftware based on the measurement of at least 300 randomly selectedsilver nanoplatelets oriented parallel to the plane of a transmissionelectron microscopy image (TEM), wherein the diameter of a silvernanoplatelet is the maximum dimension of said silver nanoplateletoriented parallel to the plane of a transmission electron microscopyimage (TEM).

The mean thickness of the silver nanoplatelets was determined bytransmission electron microscopy (TEM) based on the manual measurementof at least 50 randomly selected silver nanoplatelets orientedperpendicular to the plane of the TEM image, wherein the thickness ofthe silver nanoplatelet is the maximum thickness of said silvernanoplatelet.

B. Preparation and Characterization of Ag Nanoplatelets Dispersions D1and D2

B-1. Synthesis of Raw Material

In a 1 L double-wall glass reactor, equipped with anchor-stirrer, 365 gof de-ionized water was cooled to +2° C. 13.62 g of sodium borohydridewas added, and the mixture was cooled to −1° C. with stirring at 250rounds per minute (RPM, Solution A).

In a 0.5 L double-wall glass reactor, equipped with anchor-stirrer, 132g of deionized water and 4.8 g of MPEG-5000-thiol were combined, and themixture was stirred for 10 minutes at room temperature. 72 g of theproduct of Example A3 of WO2006074969 was added, and the resultingmixture was stirred for another 10 minutes at room temperature forhomogenization. The solution of 30.6 g of silver nitrate in 30 g ofde-ionized water was added in one portion and the mixture was stirredfor 10 minutes, resulting in an orange-brown viscous solution. To thissolution 96 g of deionized water was added, followed by addition of 3 gof Struktol SB 2080 defoamer, pre-dispersed in 36 g of de-ionized water.The resulting mixture was cooled to 0° C. with stirring at 250 RPM(Solution B).

After that, Solution B was dosed with a peristaltic pump at a constantrate over 2 h into Solution A under the liquid surface via a cooled (0°C.) dosing tube, resulting in spherical silver nanoplatelets dispersion.During pumping, the Solution A was stirred at 250 RPM.

After dosing was complete, the reaction mixture was warmed up to +5° C.within 15 minutes, and a solution of 862 mg of KCl in 10 g of deionizedwater was added in one portion, followed by addition of 9.6 g ofethylenediaminetetraacetic acid (EDTA) in 4 equal portions with 10minutes time intervals.

After addition of the last EDTA portion, the reaction mixture wasstirred for 15 minutes at +5° C., then warmed up to 35° C. over 30minutes and stirred for 1 h at this temperature. Upon this time,hydrogen evolution is completed.

3.0 mL of 30% w/w solution of ammonia in water was added, followed byaddition of 5.76 g of solid NaOH, and the mixture was stirred for 15 minat 35° C. Then 180 mL of 50% w/w hydrogen peroxide solution in waterwere dosed with a peristaltic pump at a constant rate over 4 h into thereaction mixture under the liquid surface with stirring at 250 RPM,while maintaining the temperature at 35° C. This has led to a deep bluecolored dispersion of silver nanoplatelets, which was cooled to roomtemperature. 1.23 g of compound of formula

(mixture of CAS 80584-88-9 and 80584-89-0) was added, and the mixturewas stirred for 1 h at room temperature.

B-2. Isolation and Purification of Ag Nanoplatelets

B-2a. First decantation

9.6 g of sodium dodecylsulfate was added to the reaction mixture andthen ca. 25 g of anhydrous sodium sulfate powder was added in portionswith stirring until the transmission color of the dispersion changedfrom blue to pink. Then the mixture was kept without stirring at roomtemperature for 24 h, allowing the coagulated nanoplatelets to sedimentat the bottom of the reactor.

890 g of supernatant was pumped out from the reactor with a peristalticpump, and 890 g of deionized water was added to the reactor. The mixturein reactor was stirred for 1 h at room temperature, allowing thecoagulated particles to re-disperse.

B-2b. Second Decantation

Ca. 64 g of anhydrous sodium sulfate powder was added in portions withstirring until the transmission color of the dispersion changed fromblue to yellowish-pink. Then the mixture was kept without stirring atroom temperature for 12 h, allowing the coagulated nanoplatelets tosediment at the bottom of the reactor. 990 g of supernatant was pumpedout from the reactor with a peristaltic pump, and 90 g of deionizedwater was added to the reactor. The resulting mixture was stirred for 30minutes at room temperature, allowing the coagulated particles tore-disperse.

B-2c. Ultrafiltration in Water

The resulting dispersion of Ag nanoplatelets was subjected toultrafiltration using a Millipore Amicon 8400 stirred ultrafiltrationcell. The dispersion was diluted to 400 g weight with de-ionized waterand ultrafiltered to the end volume of ca. 50 mL using apolyethersulfone (PES) membrane with 300 kDa cut-off value. Theprocedure was repeated in total 4 times to provide 60 g of Agnanoplatelets dispersion in water. After ultrafiltration was completed,0.17 g of compound of formula

(mixture of CAS 80584-88-9 and 80584-89-0) was added to the dispersion.

Ag content 28.9% w/w; yield ca. 89% based on total silver amount; Solidscontent (at 250° C.) 33.5% w/w; Purity 86% w/w of silver based on solidscontent at 250° C.

B-2d. Ultrafiltration in Isopropanol

The dispersion was further ultrafiltered in isopropanol. 60 g of Agnanoplatelets dispersion, obtained after ultrafiltration in water, wasplaced in a Millipore Amicon 8400 stirred ultrafiltration cell anddiluted to 300 g weight with isopropanol. The dispersion wasultrafiltered to the volume of ca. 50 mL using a polyethersulfone (PES)membrane with 500 kDa cut-off value. The procedure was repeated in total4 times to provide 72 g of Ag nanoplatelets dispersion in isopropanol.

Ag content 24.1% w/w; Solids content (at 250° C.) 25.7% w/w; Purity93.5% w/w of silver based on solids content at 250° C.

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of9.8*10⁻⁵ M. λ_(max)=700 nm; extinction coefficient at maximum ε=10200L/(cm*mol Ag), FWHM=340 nm.

Mean diameter of the silver nanoplatelets 93±40 nm, mean thickness ofthe silver nanoplatelets 16±2.5 nm.

B-3. Preparation of Dispersion D1

a) Surface Modification of Ag Nanoplatelets

50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained asdescribed at item B-2d, was placed in a 250 mL round-bottom flask underargon atmosphere at 23° C. 2.05 g of 5% w/w solution of carbon disulfidein absolute ethanol was added and the mixture was stirred for 5 min,followed by addition of 2.77 g of 5% w/w solution of diethanolamine inabsolute ethanol. The mixture was stirred for 1 h at 23° C., then 2.77 gof 5% w/w solution of diethanolamine in absolute ethanol was added andstirring was continued for 30 min.

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of9.8*10⁻⁵ M. λ_(max)=704 nm.

b) Solvent Switch

To the dispersion obtained in Step a), 15.0 g of ethyl3-ethoxypropionate was added. The resulting mixture was concentrated onrotary evaporator at 40 mbar pressure and 40° C. bath temperature, tillno more solvent was distilled off. The weight of the resultingdispersion was adjusted to 32.1 g by addition of ethyl3-ethoxypropionate (corresponds to the calculated total solids contentof 41.2% w/w).

B-4 Preparation of Dispersion D2

a) Surface Modification of Ag Nanoplatelets

50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained asdescribed at item B-2d, was placed in a 250 mL round-bottom flask underargon atmosphere at 23° C. 2.05 g of 5% w/w solution of carbon disulfidein absolute ethanol was added and the mixture was stirred for 5 min,followed by addition of 2.77 g of 5% w/w solution of diethanolamine inabsolute ethanol. The mixture was stirred for 1 h at 23° C., then 2.77 gof 5% w/w solution of diethanolamine in absolute ethanol was added andstirring was continued for 30 min.

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of9.8*10⁻⁵ M. λ_(max)=704 nm

b) Solvent Switch

To the dispersion, obtained in Step a), 15.0 g of7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate (CAS: 2386-87-0) was added. Theresulting mixture was concentrated on rotary evaporator at 40 mbarpressure and 40° C. bath temperature, till no more solvent was distilledoff. The weight of the resulting dispersion was adjusted to 32.1 g byaddition of 7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate (CAS: 2386-87-0) (correspondsto the calculated total solids content of 41.2% w/w).

C. Preparation of Examples (E1-E26), Comparative Examples (C1-C8) andPrinted Security Features Thereof

Description of the Ingredients of Examples (E1-E26) and ComparativeExamples (C1-C8)

TABLE 1 Ingredients Commercial name Chemical composition Ingredient(supplier) (CAS number) Polyvinylchloride Vinnol ® H14/36 85.6 wt-%PVC + 14.4 wt-% PVAc copolymer (Wacker) (9003-22-9) PolyvinylchlorideVinnol ® E22/48A 75 wt-% PVC + 25 wt-% acrylic acid/1,2- copolymer(Wacker) propanediol/butenedioic acid dibutyl ester copolymer(114653-42-8) Polyvinylchloride Vinnol ® E 15/40 A 84 wt-% PVC + 16 wt-%acrylic acid/1,2- copolymer (Wacker) propanediol copolymer (57495-45-1)Polyvinylchloride Vinnol ® H40/50 63 wt-% PVC + 37 wt-% PVAc copolymer(Wacker) (9003-22-9) Cycloaliphatic Uvacure 15007-oxabicyclo[4.1.0]hept-3-ylmethyl 7- epoxide (Allnex)oxabicyclo[4.1.0]heptane-3-carboxylate (2386-87-0) Epoxide Grilonit ®V51-63 Cyclohexane dimethanol diglycidylether (EMS Griltech)(14228-73-0) Vinylether CHDM-di (BASF)1,4-bis[(vinyloxy)methyl]cyclohexane (17351-75-6) Vinylether DVE-2(BASF) Diethyleneglycol divinylether (764-99-8) Vinylether DVE-3 (BASF)Triethylenegylcol divinylether (765-12-8) Vinylether HBVE (BASF)Hydroxybutyl vinyl ether (17832-28-9) Oxetane Curalite ™ Ox TMPO3-ethyloxetane-3-methanol (3047-32-3) (Perstorp) Oxetane Curalite ™OXPLUS 3-ethyl-3-{[(3-ethyloxetan-3- (Perstorp)yl)methoxy]methyl}oxetane (18934-00-4) Acrylate oligomer Ebecryl ® 295923 wt-% Glycerol, propoxylated, esters with (Allnex) acrylic acid(52408-84-1) + 77 wt-% 4,4′-Isopropylidenediphenol, oligomeric reactionproducts with 1-chloro-2,3-epoxypropane, esters with acrylic acid(55818-57-0) Acrylate monomer TMPTA (Allnex)2,2-bis(acryloyloxymethyl)butyl acrylate (15625-89-5) PerfluoropolyetherFluorolink ® E10H Tetrafluoroethylene, oxidized, oligomers, reduced,reactive surfactant (Solvay) methyl esters, reduced, reaction productswith ethylene oxide (162492-15-1) Average molecular weight 1700 [g/mol]Perfluoropolyether Fluorolink ® MD700 Perfluoropolyether urethanemethacrylate reactive surfactant (Solvay) (CAS not provided) Averagemolecular weight 1500 [g/mol] Perfluoropolyether Fluorolink ® F10Perfluoropolyether functionalized with phosphate anionic surfactant(Solvay) groups (200013-65-6) Average molecular weight 600-900 [g/mol]Silicone Fluorolink ® S10 Perfluoropolyether functionalized with silanegroups perfluoropolyether (Solvay) (223557-70-8) non-ionic surfactantAverage molecular weight 1750-1950 [g/mol] Silicone non-ionic BYK-330(BYK) 50% polyether modified polydimethylsiloxane in 50% surfactant2-methoxy-1-methylethyl acetate (108-65-6) Reactive silicone BYK-371(BYK) 40% acrylate polyester dimethylsiloxane in 40% non-ionicsurfactant xylene (1330-20-7) and 20% ethylbenzene (100-41-4) Reactivesilicone TEGO RAD 2300 Acrylated polyethersiloxane (CAS not provided)non-ionic surfactant (Evonik) Reactive silicone TEGO RAD 2700 Siloxanesand Silicones, di-Me, hydrogen-terminated, non-ionic surfactant (Evonik)reaction products with pentaerythritol tetraacrylate Siliconefluoroalkyl Dynasylan F-8815 Fluoroalkyl functionalized with siloxanegroups non-ionic surfactant (Evonik) (CAS not provided) Averagemolecular weight not provided by the supplier Silicone fluoroalkylDynasylan F-8261 1H,1H,2H,2H-Perfluorooctyltriethoxysilane non-ionicsurfactant (Evonik) (51851-37-7) Cationic Speedcure 976 50 wt-% mixtureof Sulfonium, diphenyl[4-(phenylthio) photoinitiator (Lambson) phenyl]-,(OC-6-11)-hexafluoroantimonate(1-) (1:1) + Sulfonium,(thiodi-4,1-phenylene)bis[diphenyl-, (OC-6- 11)-hexafluoroantimonate(1-)(1:2) (71449-78-0 and 89452-37-9) in 50% propylene carbonate (108-32-7)Cationic Speedcure 976D 35 wt-% mixture of Sulfonium, diphenyl[4-photoinitiator (Lambson) (phenylthio)phenyl]-, (OC-6-11)-hexafluoroantimonate(1-) (1:1) + Sulfonium, (thiodi-4,1-phenylene)bis[diphenyl-, (OC-6-11)- hexafluoroantimonate(1-) (1:2)(71449-78-0 and 89452-37-9) in 65 wt-% Oxirane, 2,2′-[1,4-butanediylbis(oxymethylene)]bis- (2425-79-8) Radical Omnirad 2100 92.5%ethyl phenyl(2,4,6- photoinitiator (IGM Resins)trimethylbenzoyl)phosphinate + 7.5 wt-% phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide (448-61-3) Dispersion of AgSurface stabilizing agent Amount surface nanoplatelets stabilizing agent(wt-% of the Ag nanoplatelets) Dispersion D1 ^(a)) diethanolammonium 3Dispersion D2 ^(b)) dihydroxyethyldithio- carbamate ^(a)) 41.2 wt-% Agnanoplatelets stabilized with diethanolammoniumdihydroxyethyldithiocarbamate in ethyl-3-ethoxypropionate (763-69-9)^(b)) 41.2 wt-% Ag nanoplatelets stabilized with diethanolammoniumdihydroxyethyldithiocarbamate in 7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate (Uvacure 1500, 2386-87-0)

C1. Preparation of Security Inks (E1-E8) According to the PresentInvention and Dichroic Security Features Thereof

C1a. Preparation of the Hybrid Security Inks E1-E8

Ingredients provided in Table 2a below were independently mixed anddispersed at room temperature using a Dispermat CV-3 for 10 minutes at2000 rpm so as to yield 50 g of the inks E1-E8.

TABLE 1a Composition of the UV-Vis radiation curable screen printinginks E1-E8 Amount [wt-%] Ingredient Commercial name E1 E2 E3 E4 E5 E6 E7E8 Polyvinylchloride copolymer Vinnol ® H14/36 7.4 Cycloaliphaticepoxide Uvacure ® 1500 36.2 20 20 20 20 20 20 33.7 Cationically curableCHDME-di 16.2 monomer DVE-3 16.2 HBVE 16.2 Curalite ™ OX 16.2 TMPOCuralite ™ OX 16.2 PLUS Grilonit ® V51-63 16.2 Radically curableoligomer Ebecryl 2959 4.5 Radically curable monomer TMPTA 9.1Perfluoropolyether Fluorolink E10H 2.5 surfactant Cationicphotoinitiator Speedcure 976 5.9 5.9 5.9 5.9 5.9 5.9 5.9 Speedcure 976D8.4 Free radical photoinitiator Omnirad 2100 3.1 Dispersion AgDispersion D1 ^(a)) 31.3 31.3 31.3 31.3 31.3 31.3 31.3 nanoplateletsDispersion D2 ^(b)) 31.3 Ag nanoplatelets (wt-%) 12.9 Amount of solvent(wt-%) 21.4 21.4 21.4 21.4 21.4 21.4 21.4 ^(a)) 41.2 wt-% Agnanoplatelets stabilized with diethanolammoniumdihydroxyethyldithiocarbamate in ethyl-3-ethoxypropionate (763-69-9)^(b)) 41.2 wt-% Ag nanoplatelets stabilized with diethanolammoniumdihydroxyethyldithiocarbamate in 7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate (Uvacure 1500, 2386-87-0)

C1b. Preparation of Security Features

The UV-Vis radiation curable screen printing hybrid security inks E1-E8were independently applied on pieces of transparent polymer substrate(PET Hostaphan® RN, thickness 50 μm, supplied by Putz GmbH+Co. FolienKG) using a 160 thread/cm screen (405 mesh). The printed pattern had asize of 5 cm×5 cm. 10 seconds after the printing step, the pieces ofprinted substrate were independently cured at room temperature byexposing them two times at a speed of 100 m/min to UV-Vis light under adryer from IST Metz GmbH (two lamps: iron-doped mercury lamp 200W/cm²+mercury lamp 200 W/cm²), to generate security features.

C1c. Results (Optical Properties) of Security Features

The optical properties of each security features obtained at item C1bwere independently assessed in reflection, in transmission, and visuallyusing the three tests described below. The results are summarized inTable 1c.

Reflection measurements were performed using a goniometer(Goniospektrometer Codec WI-10 5&5 by Phyma GmbH Austria). The L*a*b*values of the printed security features were determined at 0° to thenormal with an illumination angle of 22.5° on the side of thetransparent polymer substrate that was printed. The C* values (chroma,corresponding to a measure of the color intensity or color saturation)were calculated from a* and b* values according to the CIELAB (1976)color space, wherein:

C*=√{square root over ((a*)²+(b*)²)}

The C* values (reflection 22.5/0°) are displayed in Table 1c below.

Transmission measurements were carried out using a Datacolor 650spectrophotometer (parameters: integration sphere, diffuse illumination(pulse xenon D65) and 8° viewing, analyzer SP2000 with dual 256 diodearray for wavelength range of 360-700 nm, transmission sampling aperturesize of 22 mm). The C* values (transmission 8°) are displayed in Table1c below.

A visual assessment was carried out observing each security feature withthe naked eye in reflection with a diffuse source (such as the lightcoming through a window without direct sun, the observer facing the wallopposite to the window). The following colors have been observed:

-   -   Dark brown to brown colors with matte appearance and no metallic        effect;    -   Gold color (i.e. metallic yellow color) with glossy appearance        and metallic effect. The metallic effect appears for a chroma        value C* in reflection 22.5/0° higher than about 20.

A visual assessment was also carried out observing each security featurewith the naked eye in transmission. The following colors have beenobserved:

-   -   Dull blue: the blue coloration is weak (but visible);    -   Blue (chroma value C* in transmission 8° higher than or equal to        about 20) to deep blue (chroma value C* in transmission 8°        higher than or equal to 30): the blue coloration is intense to        very intense.

As shown in Table 1c, the security features obtained from inks E1-E8according to the invention exhibited gold color in reflection and blueto deep blue color in transmission.

TABLE 1c Color properties of security features obtained from inks E1-E8E1 E2 E3 E4 E5 E6 E7 E8 C* (reflection 22.5/0°) 31 35 40 34 24 25 28 24C* (transmission 8°) 36 36 40 42 43 39 48 41 Color (reflection) GoldGold Gold Gold Gold Gold Gold Gold Color (transmission) Deep Deep DeepDeep Deep Deep Deep Deep blue blue blue blue blue blue blue blue

As attested by Table 1c, solvent-containing hybrid security inks E1-E7according to the present invention and solvent-free hybrid security inkE8 according to the present invention comprising either a mixture ofcycloaliphatic epoxide and radically curable monomers/oligomers, or amixture of cycloaliphatic epoxide, radically curable monomers/oligomersand cationically curable monomers provide security features withexcellent visual aspect and high chroma values C* both in reflectedlight and in transmitted light.

C2. Preparation of Security Inks (E9-E15) According to the PresentInvention and Dichroic Security Features Thereof

C2a. Preparation of the Cationically Curable Security Inks E9-E15

Ingredients provided in Table 2a were mixed and dispersed at roomtemperature using a Dispermat CV-3 for 10 minutes at 2000 rpm so as toyield 50 g of each ink E9-E15.

TABLE 2a Composition of the UV-Vis radiation curable screen printinginks E9-E15. Amount [wt-%] Ingredient Commercial name E9 E10 E11 E12 E13E14 E15 Polyvinylchloride copolymer Vinnol ® H14/36 7.4 Cycloaliphaticepoxide Uvacure ® 1500 47.3 31.1 31.1 31.1 31.1 23.6 31.1 Cationicallycurable monomer CHDM-di 16.2 16.2 DVE-3 16.2 23.7 Curalite ™ OX PLUS16.2 Grilonit ® V51-63 16.2 Perfluoropolyether surfactant FluorolinkE10H 2.5 Cationic photoinitiator Speedcure 976D 11.5 Dispersion Agnanoplatelets Dispersion D1 ^(a)) 31.3 31.3 31.3 Dispersion D2 ^(b))31.3 31.3 31.3 31.3 Ag nanoplatelets (wt-%) 12.9 Amount of solvent(wt-%) 18.4 18.4 18.4 ^(a)) 41.2 wt-% Ag nanoplatelets stabilized withdiethanolammonium dihydroxyethyldithiocarbamate inethyl-3-ethoxypropionate (763-69-9) ^(b)) 41.2 wt-% Ag nanoplateletsstabilized with diethanolammonium dihydroxyethyldithiocarbamate in7-oxabicyclo[4.1.0]hept-3-ylmethyl7-oxabicyclo[4.1.0]heptane-3-carboxylate (Uvacure 1500, 2386-87-0)

C2b. Preparation of Security Features

The UV-Vis radiation curable screen printing inks E9-E15 wereindependently applied on pieces of transparent polymer substrate (PETHostaphan® RN, thickness 50 μm, supplied by Putz GmbH+Co. Folien KG)using a 160 thread/cm screen (405 mesh). The printed pattern had a sizeof 5 cm×5 cm. 10 seconds after the printing step, the pieces of printedsubstrate were independently cured at room temperature by exposing themtwo times at a speed of 100 m/min to UV-Vis light under a dryer from ISTMetz GmbH (two lamps: iron-doped mercury lamp 200 W/cm²+mercury lamp 200W/cm²), to generate security features.

C2c. Results (Optical Properties) of Security Features

The optical properties of the security features obtained at item C2bwere independently assessed in reflection, in transmission, and visuallyusing the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection22.5/0° and transmission 8°) exhibited by the security features preparedusing the comparative inks E9-E15 according to the present invention aredisplayed in Table 2c (below).

TABLE 2c Color properties of security features obtained from inks E9-E15E9 E10 E11 E12 E13 E14 E15 C* (reflection 22.5/0°) 22 33 23 27 23 29 28C* (transmission 8°) 32 39 33 43 43 43 34 Color (reflection) Gold GoldGold Gold Gold Gold Gold Color (transmission) Deep Deep Deep Deep DeepDeep Deep blue blue blue blue blue blue blue

As attested by Table 2c, cationically curable solvent-free security inksE9-E12 according to the present invention and cationically curablesolvent-containing security inks E13-E15 according to the presentinvention comprising either cycloaliphatic epoxide, or cycloaliphaticepoxide and other cationically curable monomers provide securityfeatures with excellent visual aspect and high chroma values C* both inreflected light and in transmitted light.

C3. Study of the Influence of the Surfactant on the Optical PropertiesExhibited by the Security Feature (Comparative Inks C1-C7 and InksE16-E18 According to the Present Invention)

To evaluate the influence of the surfactant on the optical propertiesexhibited by the security feature, comparative inks C1-C7 and inksE16-E18 according to the invention were prepared.

C3a. Preparation of the ink C1-C7 and E16-E18

Ingredients provided in Table 3a were mixed and dispersed at roomtemperature using a Dispermat CV-3 for 10 minutes at 2000 rpm so as toyield 50 g of each ink C1-C7 and E16-E18.

TABLE 3a Composition of the UV-Vis radiation curable screen printinginks C1-C7 and E18-E20. Amount [wt-%] Ingredient Commerical name E16 E17E18 C1 C2 C3 C4 C5 C6 C7 Polyvinylchlonde copolymer Vinnol ® H14/36 7.4Cycloaliphatic epoxide Uvacure ® 1500 20 Cationically curable monomerCHDM-di 16.2 Radically curable oligomer Ebecryl 2959 4.5 Radicallycurable monomer TMPTA 9.1 Perfluoropolyether surfactant Fluorolink E10H2.5 Fluorolink MD700 2.5 Fluorolink S10 2.5 Fluorolink F10 2.5 Siliconesurfactant BYK 330 2.5 BYK 371 2.5 Tego Rad 2300 2.5 Tego Rad 2700 2.5Fluoro silicone surfactant Dynasdan F-8815 25 Dynasilan F-8261 2.5Cationic photoinitiator Speedcure 976 5.9 Free radical photoinitiatorOmnirad 2100 3.1 Dispersion Ag nanoplatelets Dispersion D1^(a)) 31.3

C3b. Preparation of Security Features

The UV-Vis radiation curable screen printing inks C1-C7 and E16-E18 wereindependently applied on pieces of transparent polymer substrate (PETHostaphang RN, thickness 50 μm, supplied by Putz GmbH+Co. Folien KG)using a 160 thread/cm screen (405 mesh). The printed pattern had a sizeof 5 cm×5 cm. 10 seconds after the printing step, the pieces of printedsubstrate were independently cured at room temperature by exposing themtwo times at a speed of 100 in/min to UV-Vis light under a dryer fromIST Metz GmbH (two lamps: iron-doped mercury lamp 200 W/cm²+mercury lamp200 W/cm²), to generate security features.

C3c. Results (Optical Properties) of Security Features

The optical properties of the security features at item C3b wereindependently assessed in reflection, in transmission, and visuallyusing the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection22.5/00 and transmission 80) exhibited by the security features preparedusing the inks C1-C7 and E16-E18 are displayed in Table 3c below.

TABLE 3c Color properties of security features obtained from inks C1-C7and E16-E18 E16 E17 E18 C1 C2 C3 C4 C5 C6 C7 Surfactant FluorolinkFluorolink Fluorolink Fluorolink BYK BYK Tego Tego Dynasilan DynasilanE10H MD700 S10 F10 330 371 Rad 2300 Rad 2700 F-8815 F-8261 C*(reflection 22.5/0°) 29 22 20 4 4 2 2 4 2 3 C* (transmission 8°) 23 2728 14 27 13 12 26 4 24 Color (reflection) Gold Gold Gold Dark Dark DarkDark Dark Dark Dark brown brown brown brown brown brown brown Color(transmission) Blue Blue Blue Dull blue Blue Dull blue Dull blue BlueDull blue Blue

As shown in Table 3c the security features obtained from an inkaccording to the invention comprising a perfluoropolyether surfactantfunctionalized with hydroxyl groups (Fluorolink E10H: ink E16),(meth)acrylate groups (Fluorolink MD700: ink E17), or silane groups(Fluorolink S10: ink E18), exhibit gold color in reflection and deepblue color in transmission. By comparison, the security featuresobtained from inks comprising perfluoropolyether surfactants lackingsaid functional groups (Fluorolink F10: comparative ink C1), orsurfactants lacking the perfluoropolyether chain, such as BYK 330(comparative ink C2), BYK 371 (comparative ink C3), TEGO RAID 2300(comparative ink C4), TEGO RAD 2700 (comparative ink C5), DynasylanF8815 (comparative ink C6) and Dynasylan F8261 (comparative ink C7)exhibit a dull blue to blue color in transmission, but a dark brown tobrown color with low chroma value in reflection. A dark brown to browncolor with low chroma value in reflection is not eye-catching andtherefore, not suitable for a dichroic security feature for securing avalue document.

C4. Study of the Influence of the Amount of the PerfluoropolyetherSurfactant on the Optical Properties Exhibited by the Security Feature(Comparative Ink C8 and Inks E19-E23 According to the Present Invention)

To evaluate the influence of the of the amount of the perfluoropolyethersurfactant as described herein on the optical properties exhibited bythe security feature, inks C8 and E19-E23 according to the inventionwere prepared as described below.

C4a. Preparation of the Ink C8 and E19-E23

Ingredients provided in Table 4a were mixed and dispersed at roomtemperature using a Dispermat CV-3 for 10 minutes at 2000 rpm so as toyield 50 g of each ink C8 and E19-E23.

TABLE 4a Composition of the UV-Vis radiation curable screen printinginks C8 and E19-E23. Amount [wt-%] Ingredient Commercial name C8 E19 E20E21 E22 E23 Polyvinylchloride Vinnol ® H14/36 7.4 copolymerCycloaliphatic Uvacure ® 1500 22 22.45 22 21.5 20 20 epoxideCationically-curable CHDM-di 16.7 16.2 16.2 16.2 16.2 13.7 monomerRadically curable Ebecryl 2959 4.5 oligomer Radically curable TMPTA 9.1monomer Perfluoropolyether Fluorolink E10H 0 0.05 0.5 1 2.5 5 surfactantCationic Speedcure 976 5.9 photoinitiator Free radical Omnirad 2100 3.1photoinitiator Dispersion Ag Dispersion D1 ^(a)) 31.3 nanoplatelets^(a)) 41.2 wt-% Ag nanoplatelets stabilized with diethanolammoniumdihydroxyethyldithiocarbamate in ethyl-3-ethoxypropionate (763-69-9)

C4b. Preparation of Security Features

The UV-Vis radiation curable screen printing inks C8 and E19-E23 wereindependently applied on pieces of transparent polymer substrate (PETHostaphan® RN, thickness 50 μm, supplied by Putz GmbH+Co. Folien KG)using a 160 thread/cm screen (405 mesh). The printed pattern had a sizeof 5 cm×5 cm. About 10 seconds after the printing step, the pieces ofprinted substrate were independently cured at room temperature byexposing them two times at a speed of 100 m/min to UV-Vis light under adryer from IST Metz GmbH (two lamps: iron-doped mercury lamp 200W/cm²+mercury lamp 200 W/cm²), to generate security features.

C4c. Results (Optical Properties) of Security Features

The optical properties of the security features obtained at item C4bwere independently assessed in reflection, in transmission, and visuallyusing the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection22.5/0° and transmission 8°) exhibited by the security features preparedusing the inks C8 and E19-E23 are displayed in Table 4c below.

TABLE 4c Color properties of security features obtained from inks C8 andE19-E23 C8 E19 E20 E21 E22 E23 Amount of surfactant 0 0.05 0.5 1 2.5 5C* (reflection 22.5/0°) 2 32 33 33 35 31 C* (transmission 8°) 24 47 4340 36 42 Color (reflection) Dark Gold Gold Gold Gold Gold brown Color(transmission) Blue Deep Deep Deep Deep Deep blue blue blue blue blue

As shown in Table 4c, the use of an amount from about 0.05 wt-% to about5 wt-% of a perfluoropolyether surfactant as described herein (such asFluorolink E10H used in the inks E19-E23) ensures that security featuresshowing metallic yellow color with high chroma values in reflection anddeep blue color in transmission are obtained (inks E19-E23). Bycomparison, the security feature obtained in the experiment conductedwith an ink C8 containing no surfactant exhibits a dark brown to browncolor with low chroma value in reflection. Such color is noteye-catching for the layperson and cannot be used as security featurefor securing a value document.

C5. Study of the Influence of the Type of Polyvinylchloride Copolymer onthe Optical Properties Exhibited by the Security Feature (Inks E1 andE24-E26 According to the Invention)

To evaluate the influence of the type of polyvinylchloride copolymer onthe optical properties exhibited by the security feature, inks E1 andE24-E26 according to the invention were prepared as described below.

C5a. Preparation of the Inks E1 and E24-E26

Ingredients provided in Table 5a were mixed and dispersed at roomtemperature using a Dispermat CV-3 for 10 minutes at 2000 rpm so as toyield 50 g of each ink E1 and E24-E26.

TABLE 5a Composition of the UV-Vis radiation curable screen printinginks E1 and E24-E26. Amount wt-% Ingredient Commercial name E1 E24 E25E26 Polyvinylchloride Vinnol ® H14/36 ^(a)) 7.4 copolymer Vinnol ®H40/50 ^(b)) 7.4 Vinnol ® E15/40A ^(c)) 7.4 Vinnol ® E22/48A ^(d)) 7.4Cycloaliphatic Uvacure ® 1500 36.2  epoxide Radically curable Ebecryl2959 4.5 oligomer Radically curable TMPTA 9.1 monomer PerfluoropolyetherFluorolink E10H 2.5 reactive surfactant Cationic Speedcure 976 5.9photoinitiator Free radical Omnirad 2100 3.1 photoinitiator DispersionAg Dispersion D1 ^(e)) 31.3  nanoplatelets Percentage of PVC in 85.6 6384 75 polyvinylchloride copolymer [wt-%] ^(a))polyvinylchloride/polyvinylacetate (85.6%/14.4%) (9003-22-9), K value 35± 1, molecular mass 3-4 · 10⁴ Dalton (Wacker, size-exclusionchromatography) ^(b)) polyvinylchloride/polyvinylacetate (63%/37%)(9003-22-9), K value 50 ± 1, molecular mass 6-8 · 10⁴ Dalton ^(c))polyvinylchloride/(acrylic acid/1,2-propanediol copolymer) (57495-45-1)(84%/16%), K value 39 ± 1, molecular mass 4-5 · 10⁴ Dalton ^(d))polyvinylchloride/(acrylic acid/1,2-propanediol/butenedioic acid dibutylester copolymer) (114653-42-8) (75%/25%), K value 48 ± 1, molecular mass6-8 · 10⁴ Dalton ^(e)) 41.2 wt-% Ag nanoplatelets stabilized withdiethanolammonium dihydroxyethyldithiocarbamate inethyl-3-ethoxypropionate (763-69-9)

C5b. Preparation of Security Features

The UV-Vis radiation curable screen printing inks E1 and E24-E26 wereindependently applied on pieces of transparent polymer substrate (PETHostaphan® RN, thickness 50 μm, supplied by Putz GmbH+Co. Folien KG)using a 160 thread/cm screen (405 mesh). The printed pattern had a sizeof 5 cm×5 cm. 10 seconds after the printing step, the pieces of printedsubstrate were independently cured at room temperature by exposing themtwo times at a speed of 100 m/min to UV-Vis light under a dryer from ISTMetz GmbH (two lamps: iron-doped mercury lamp 200 W/cm²+mercury lamp 200W/cm²), to generate security features.

C5c. Results (Optical Properties) of Security Features

The optical properties of the security features obtained at item C5bwere independently assessed in reflection, in transmission, and visuallyusing the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection22.5/0° and transmission 8°) exhibited by the security features preparedusing the inks E1 and E24-E26 are displayed in Table 5c below.

TABLE 5c Color properties of security features obtained from inks E1 andE24-E26 E1 E24 E25 E26 Percentage of PVC in 85.6 63 84 75polyvinylchloride copolymer [wt-%] C* (reflection 22.5/0°) 31 20 29 30C* (transmission 8°) 36 29 43 41 Color (reflection) Gold Gold Gold GoldColor (transmission) Deep blue Blue Deep blue Deep blue

As attested by the optical properties of security features shown inTable 5c, security inks as claimed herein comprising a polyvinylchloride copolymer which contains at least about 60 wt-%, preferably atleast about 63 wt-% of vinyl chloride, provide security featureexhibiting blue to deep blue color in reflection and a metallic yellowcolor in reflection.

C6. Study of the Stability of the UV-Vis Radiation Curable Security InkE2 According to the Invention Via an Accelerated Ageing Experiment

To evaluate the stability upon time of an ink according to theinvention, 10 g of ink E2 (described in Table 1a) were placed in a cup(60 ml white polypropylene cup for SpeedMixer™ available at Hauschild &Co. KG), which was hermetically sealed and stored for five months at atemperature of 60° C. in a laboratory oven (Kendro Laboratory Products,T6060). The freshly prepared ink E2 was used as a comparison standard.Each month, the sample of ink E2 stored in the oven was cooled at roomtemperature for 6 hours, and subsequently applied on a piece oftransparent polymer substrate (PET Hostaphan® RN, thickness 50 μm,supplied by Putz GmbH+Co. Folien KG) using a 160 thread/cm screen (405mesh). The printed pattern had a size of 5 cm×5 cm. 10 seconds after theprinting step, the piece of printed substrate was cured at roomtemperature by two times exposure at a speed of 100 m/min to UV-Vislight under a dryer from IST Metz GmbH (two lamps: iron-doped mercurylamp 200 W/cm²+mercury lamp 200 W/cm²), to generate a security feature.The optical properties of the security feature obtained each month wereindependently assessed in reflection, and visually using the testsdescribed at item C1c. Table 6 summarizes the color in reflection andtransmission and the C* values (reflection 22.5/0°) exhibited by thesecurity features.

TABLE 6 Color properties of security Time interval (months) 0^(a)) 1 2 34 5 C* (reflection 22.5/0°) 35 37 34 35 35 34 Color (reflection) GoldGold Gold Gold Gold Gold Color (transmission) Deep Deep Deep Deep DeepDeep blue blue blue blue blue blue ^(a))the security feature wasmanufactured with the freshly prepared ink E2 according to the presentinvention.

As attested by the optical properties of security features shown inTable 6, the ink E2 according to the invention remains stable over anextended period of time at high temperature, which is an indication ofoutstanding shelf stability at room temperature.

1. A UV-Vis radiation curable security ink for producing a securityfeature exhibiting a blue color upon viewing in transmitted light and ametallic yellow color upon viewing in incident light, wherein said inkcomprises: a) from about 7.5 wt-% to about 20 wt-% of silvernanoplatelets having a mean diameter in the range of 50 to 150 nm with astandard deviation of less than 60%, a mean thickness in the range of 5to 30 nm with a standard deviation of less than 50%, and a mean aspectratio higher than 2.0, wherein the mean diameter is determined bytransmission electron microscopy and the mean thickness is determined bytransmission electron microscopy, and wherein the silver nanoplateletsbear a surface stabilizing agent of general formula (I)

wherein the residue R^(A) is a C₂-C₄alkyl group substituted with ahydroxy group; the residue R^(B) is selected from a C₁-C₄ alkyl group,and a C₂-C₄alkyl group substituted with a hydroxy group; and Cat⁺ is anammonium cation of general formula ⁺NH₂R^(C)R^(D), wherein the residueR^(C) is a C₂-C₄alkyl group substituted with a hydroxy group; and theresidue R^(D) is selected from a C₁-C₄alkyl group, and a C₂-C₄alkylgroup substituted with a hydroxy group; b) from about 45 wt-% to about80 wt-% of either a cycloaliphatic epoxide, or a mixture of acycloaliphatic epoxide and one or more UV-Vis radiation curablecompounds; c) one or more cationic photoinitiators; d) aperfluoropolyether surfactant functionalized with one or more functionalgroups selected from the group consisting of: hydroxyl, acrylate,methacrylate, and trialkoxysilyl; e) from about 3 wt-% to about 12 wt-%of a polyvinyl chloride copolymer containing at least 60 wt-% of vinylchloride; and optionally f) up to about 25 wt-% of an organic solvent;the weight percents being based on the total weight of the UV-Visradiation curable security ink.
 2. The UV-Vis radiation curable securityink according to claim 1, wherein the security ink is selected from ascreen-printing security ink, a rotogravure security ink, and aflexography security ink.
 3. The UV-Vis radiation curable security inkaccording to claim 1, wherein the mean diameter of the silvernanoplatelets is in the range of 70 to 120 nm with the standarddeviation being less than 50%, the mean thickness of the silvernanoplatelets is in the range of 8 to 25 nm with the standard deviationbeing less than 30%, and the mean aspect ratio of the silvernanoplatelets is higher than 2.5.
 4. The UV-Vis radiation curablesecurity ink according to claim 1, wherein the surface stabilizing agentof general formula (I) is present in an amount from about 0.5% to about5% of the weight percent (wt-%) of the silver nanoplatelets of a). 5.The UV-Vis radiation curable security ink according to claim 1, whereinthe residues R^(A) and R^(B) are independently of each other aC₂-C₄alkyl group substituted with a hydroxy group.
 6. The UV-Visradiation curable security ink according to claim 1, wherein the silvernanoplatelets bear a further surface stabilizing agent of generalformula (II)

wherein

indicates the bond to the silver; R¹ is H, C₁-C₁₈alkyl, phenyl,C₁-C₈alkylphenyl, or CH₂COOH; R², R³, R⁴, R⁵, R⁶ and R⁷ areindependently of each other H, C₁-C₈alkyl, or phenyl; Y is O, or NR⁸; R⁸is H, or C₁-C₈alkyl; k1 is an integer in the range of from 1 to 500; k2and k3 are independently of each other 0, or integers in the range offrom 1 to 250; k4 is 0, or 1; and k5 is an integer in the range of from1 to
 5. 7. The UV-Vis radiation curable security ink according to claim1, wherein the silver nanoplatelets bear a further surface stabilizingagent of general formula (III)

wherein R^(17a), R^(17b) and R^(17c) are independently of each other H,or methyl; R^(18a) and R^(18b) are H, or methyl; R^(19a) is saturated orunsaturated, linear or branched chain alkyl with 1-22 carbon atoms;R^(19b) is R_(c)—[O—CH₂—CH₂—]_(c)—O—; R^(19c) is

—C(═O)—NH—(CH₂)_(y)N⁺HR¹⁵R¹⁶An⁻; wherein An⁻ is an anion of a monovalentorganic, or inorganic acid; y is an integer from 2 to 10; R¹⁵ issaturated or unsaturated, linear or branched chain alkyl with 1-22carbon atoms, R¹⁶ is saturated or unsaturated, linear or branched chainalkyl with 1-22 carbon atoms, R_(c) is saturated or unsaturated, linearor branched chain alkyl with 1-22 carbon atoms, or alkylaryl ordialkylaryl with up to 24 carbon atoms and c is 1 to 150, and y1, y2 andy3 are independently of each other integers from 1 to
 200. 8. The UV-Visradiation curable security ink according to claim 1, wherein the silvernanoplatelets bear a further surface stabilizing agent of generalformula (IV)

wherein R⁹ is a hydrogen atom, or a group of formula —CHR¹¹—N(R¹²)(R¹³);R¹⁰ is a hydrogen atom, a halogen atom, a C₁-C₈alkoxy group, or aC₁-C₈alkyl group; R¹¹ is H, or C₁-C₈alkyl; and R¹² and R¹³ areindependently of each other a C₁-C₈alkyl, a C₁-C₈alkyl group substitutedby a hydroxy group, or a group of formula —[(CH₂CH₂)—O]_(n1)—CH₂CH₂—OH,wherein n1 is 1 to
 5. 9. The UV-Vis radiation curable security inkaccording to claim 1, wherein the one or more UV-Vis radiation curablecompounds comprise one or more cationically curable monomers selectedfrom the group consisting of: vinyl ethers, propenyl ethers, cyclicethers other than a cycloaliphatic epoxide, lactones, cyclic thioethers,vinyl thioethers, propenyl thioethers, hydroxyl-containing compounds,and mixtures thereof.
 10. The UV-Vis radiation curable security inkaccording to claim 1, wherein the one or more UV-Vis radiation curablecompounds comprise one or more radically curable monomers and/oroligomers, and the UV-Vis radiation curable ink further comprises g) oneor more free radical photoinitiators.
 11. The UV-Vis radiation curablesecurity ink according to claim 1, wherein the UV-Vis radiation curablesecurity ink comprises an amount of perfluoropolyether surfactant fromabout 0.025 wt-% to about 5 wt-%, wherein the weight percents are basedon the total weight of the UV-Vis radiation curable security ink. 12.The UV-Vis radiation curable security ink according to claim 1, whereinthe UV-Vis radiation curable security ink is solvent-free.
 13. A processfor producing a security feature for securing a value document, whereinsaid security feature exhibits a blue color upon viewing in transmittedlight and a metallic yellow color upon viewing in incident light, saidprocess comprising the following steps: A) printing the UV-Vis radiationcurable security ink according to claim 1 on a transparent or partiallytransparent region of a substrate of a value document to provide an inklayer; and B) UV-Vis curing the ink layer obtained at step A) to formthe security feature.
 14. The process according to claim 13, wherein thevalue document is selected from the group consisting of banknotes,deeds, tickets, checks, vouchers, fiscal stamps, agreements, identitydocuments such as passports, identity cards, visas, driving licenses,bank cards, credit cards, transactions cards, access documents, andcards, entrance tickets, public transportation tickets, academicdiploma, and academic titles.
 15. The process according to claim 13,wherein the printing is performed by screen printing, rotogravure, orflexography.